diff --git a/ash/BUILD.gn b/ash/BUILD.gn
index 7ce5e923e1e7ff..90dd25539b6598 100644
--- a/ash/BUILD.gn
+++ b/ash/BUILD.gn
@@ -34,6 +34,7 @@ component("ash") {
     "accelerators/accelerator_controller.h",
     "accessibility/accessibility_controller.h",
     "accessibility/accessibility_delegate.h",
+    "accessibility/ax_host_service.h",
     "accessibility/focus_ring_controller.h",
     "app_list/app_list_controller_impl.h",
     "detachable_base/detachable_base_handler.h",
@@ -131,6 +132,7 @@ component("ash") {
     "accessibility/accessibility_observer.h",
     "accessibility/accessibility_panel_layout_manager.cc",
     "accessibility/accessibility_panel_layout_manager.h",
+    "accessibility/ax_host_service.cc",
     "accessibility/default_accessibility_delegate.cc",
     "accessibility/default_accessibility_delegate.h",
     "accessibility/focus_ring_controller.cc",
@@ -1687,6 +1689,7 @@ test("ash_unittests") {
     "accessibility/accessibility_focus_ring_group_unittest.cc",
     "accessibility/accessibility_highlight_controller_unittest.cc",
     "accessibility/accessibility_panel_layout_manager_unittest.cc",
+    "accessibility/ax_host_service_unittest.cc",
     "accessibility/key_accessibility_enabler_unittest.cc",
     "accessibility/touch_accessibility_enabler_unittest.cc",
     "accessibility/touch_exploration_controller_unittest.cc",
diff --git a/ash/accessibility/accessibility_delegate.h b/ash/accessibility/accessibility_delegate.h
index c59d0e6b2dcdd0..0a33e6d344c7c2 100644
--- a/ash/accessibility/accessibility_delegate.h
+++ b/ash/accessibility/accessibility_delegate.h
@@ -5,9 +5,15 @@
 #ifndef ASH_ACCESSIBILITY_ACCESSIBILITY_DELEGATE_H_
 #define ASH_ACCESSIBILITY_ACCESSIBILITY_DELEGATE_H_
 
+#include <vector>
+
 #include "ash/ash_export.h"
-#include "base/time/time.h"
-#include "ui/accessibility/ax_enums.mojom.h"
+#include "ui/accessibility/ax_tree_id.h"
+#include "ui/accessibility/ax_tree_update.h"
+
+namespace ui {
+struct AXEvent;
+}
 
 namespace ash {
 
@@ -36,6 +42,14 @@ class ASH_EXPORT AccessibilityDelegate {
   // is not saved, return a negative value.
   virtual double GetSavedScreenMagnifierScale() = 0;
 
+  // Automation API support for remote mojo apps.
+  // TODO(jamescook): Convert to mojo API.
+  virtual void DispatchAccessibilityEvent(
+      const ui::AXTreeID& tree_id,
+      const std::vector<ui::AXTreeUpdate>& updates,
+      const ui::AXEvent& event) = 0;
+  virtual void DispatchTreeDestroyedEvent(const ui::AXTreeID& tree_id) = 0;
+
   // NOTE: Prefer adding methods to AccessibilityController, see class comment.
 };
 
diff --git a/chrome/browser/chromeos/accessibility/ax_host_service.cc b/ash/accessibility/ax_host_service.cc
similarity index 70%
rename from chrome/browser/chromeos/accessibility/ax_host_service.cc
rename to ash/accessibility/ax_host_service.cc
index 6b1282378687ae..00ac720ca6a561 100644
--- a/chrome/browser/chromeos/accessibility/ax_host_service.cc
+++ b/ash/accessibility/ax_host_service.cc
@@ -2,19 +2,19 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "chrome/browser/chromeos/accessibility/ax_host_service.h"
+#include "ash/accessibility/ax_host_service.h"
 
 #include "ash/accessibility/accessibility_controller.h"
+#include "ash/accessibility/accessibility_delegate.h"
 #include "ash/shell.h"
 #include "base/bind.h"
-#include "chrome/browser/extensions/api/automation_internal/automation_event_router.h"
-#include "chrome/common/extensions/chrome_extension_messages.h"
 #include "services/service_manager/public/cpp/connector.h"
 #include "services/service_manager/public/cpp/service_context.h"
 #include "ui/accessibility/ax_event.h"
-#include "ui/aura/env.h"
 #include "ui/views/mus/ax_remote_host.h"
 
+namespace ash {
+
 AXHostService* AXHostService::instance_ = nullptr;
 
 bool AXHostService::automation_enabled_ = false;
@@ -23,12 +23,8 @@ AXHostService::AXHostService() {
   // AX tree ID is automatically assigned.
   DCHECK(!tree_id().empty());
 
-  // ash::Shell may not exist in tests.
-  if (ash::Shell::HasInstance()) {
-    // TODO(jamescook): Eliminate this when tree ID assignment is handed in ash.
-    ash::Shell::Get()->accessibility_controller()->set_remote_ax_tree_id(
-        tree_id());
-  }
+  // TODO(jamescook): Eliminate this when multiple remote trees are supported.
+  Shell::Get()->accessibility_controller()->set_remote_ax_tree_id(tree_id());
 
   DCHECK(!instance_);
   instance_ = this;
@@ -71,16 +67,12 @@ void AXHostService::HandleAccessibilityEvent(
     const std::vector<ui::AXTreeUpdate>& updates,
     const ui::AXEvent& event) {
   CHECK_EQ(tree_id, this->tree_id());
-  ExtensionMsg_AccessibilityEventBundleParams event_bundle;
-  event_bundle.tree_id = tree_id;
-  for (const ui::AXTreeUpdate& update : updates)
-    event_bundle.updates.push_back(update);
-  event_bundle.events.push_back(event);
-  event_bundle.mouse_location = aura::Env::GetInstance()->last_mouse_location();
-
-  // Forward the tree updates and the event to the accessibility extension.
-  extensions::AutomationEventRouter::GetInstance()->DispatchAccessibilityEvents(
-      event_bundle);
+  // Use an in-process delegate back to Chrome for efficiency in classic ash.
+  // For mash we'll need to pass around a mojo interface pointer such that a
+  // remote app can talk directly to the browser process and not ping-pong
+  // through the ash process.
+  Shell::Get()->accessibility_delegate()->DispatchAccessibilityEvent(
+      tree_id, updates, event);
 }
 
 void AXHostService::PerformAction(const ui::AXActionData& data) {
@@ -104,6 +96,7 @@ void AXHostService::NotifyAutomationEnabled() {
 }
 
 void AXHostService::OnRemoteHostDisconnected() {
-  extensions::AutomationEventRouter::GetInstance()->DispatchTreeDestroyedEvent(
-      tree_id(), nullptr /* browser_context */);
+  Shell::Get()->accessibility_delegate()->DispatchTreeDestroyedEvent(tree_id());
 }
+
+}  // namespace ash
diff --git a/chrome/browser/chromeos/accessibility/ax_host_service.h b/ash/accessibility/ax_host_service.h
similarity index 85%
rename from chrome/browser/chromeos/accessibility/ax_host_service.h
rename to ash/accessibility/ax_host_service.h
index 9d2fa2a7fdf757..b0809cde04753d 100644
--- a/chrome/browser/chromeos/accessibility/ax_host_service.h
+++ b/ash/accessibility/ax_host_service.h
@@ -2,26 +2,30 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_AX_HOST_SERVICE_H_
-#define CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_AX_HOST_SERVICE_H_
+#ifndef ASH_ACCESSIBILITY_AX_HOST_SERVICE_H_
+#define ASH_ACCESSIBILITY_AX_HOST_SERVICE_H_
 
 #include <memory>
 
+#include "ash/ash_export.h"
 #include "mojo/public/cpp/bindings/binding_set.h"
 #include "mojo/public/cpp/bindings/interface_ptr_set.h"
 #include "services/service_manager/public/cpp/binder_registry.h"
 #include "services/service_manager/public/cpp/service.h"
 #include "ui/accessibility/ax_host_delegate.h"
+#include "ui/accessibility/ax_tree_id.h"
 #include "ui/accessibility/mojom/ax_host.mojom.h"
 
+namespace ash {
+
 // Forwards accessibility events from clients in other processes that use aura
 // and views (e.g. the Chrome OS keyboard shortcut_viewer) to accessibility
 // extensions. Renderers, PDF, etc. use a different path. Created when the first
 // client connects over mojo. Implements AXHostDelegate by routing actions over
 // mojo to the remote process.
-class AXHostService : public service_manager::Service,
-                      public ax::mojom::AXHost,
-                      public ui::AXHostDelegate {
+class ASH_EXPORT AXHostService : public service_manager::Service,
+                                 public ax::mojom::AXHost,
+                                 public ui::AXHostDelegate {
  public:
   AXHostService();
   ~AXHostService() override;
@@ -66,4 +70,6 @@ class AXHostService : public service_manager::Service,
   DISALLOW_COPY_AND_ASSIGN(AXHostService);
 };
 
-#endif  // CHROME_BROWSER_CHROMEOS_ACCESSIBILITY_AX_HOST_SERVICE_H_
+}  // namespace ash
+
+#endif  // ASH_ACCESSIBILITY_AX_HOST_SERVICE_H_
diff --git a/chrome/browser/chromeos/accessibility/ax_host_service_unittest.cc b/ash/accessibility/ax_host_service_unittest.cc
similarity index 91%
rename from chrome/browser/chromeos/accessibility/ax_host_service_unittest.cc
rename to ash/accessibility/ax_host_service_unittest.cc
index 43ac4b4830a76f..4ecc260bccde73 100644
--- a/chrome/browser/chromeos/accessibility/ax_host_service_unittest.cc
+++ b/ash/accessibility/ax_host_service_unittest.cc
@@ -2,8 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "chrome/browser/chromeos/accessibility/ax_host_service.h"
+#include "ash/accessibility/ax_host_service.h"
 
+#include "ash/test/ash_test_base.h"
 #include "base/bind.h"
 #include "base/macros.h"
 #include "base/test/scoped_task_environment.h"
@@ -11,6 +12,7 @@
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/accessibility/mojom/ax_host.mojom.h"
 
+namespace ash {
 namespace {
 
 class TestAXRemoteHost : ax::mojom::AXRemoteHost {
@@ -51,16 +53,7 @@ class TestAXRemoteHost : ax::mojom::AXRemoteHost {
   DISALLOW_COPY_AND_ASSIGN(TestAXRemoteHost);
 };
 
-class AXHostServiceTest : public testing::Test {
- public:
-  AXHostServiceTest() = default;
-  ~AXHostServiceTest() override = default;
-
- private:
-  base::test::ScopedTaskEnvironment scoped_task_enviroment_;
-
-  DISALLOW_COPY_AND_ASSIGN(AXHostServiceTest);
-};
+using AXHostServiceTest = AshTestBase;
 
 TEST_F(AXHostServiceTest, AddClientThenEnable) {
   AXHostService service;
@@ -120,3 +113,4 @@ TEST_F(AXHostServiceTest, PerformAction) {
 }
 
 }  // namespace
+}  // namespace ash
diff --git a/ash/accessibility/default_accessibility_delegate.cc b/ash/accessibility/default_accessibility_delegate.cc
index f57af835c79bb7..f4b5ec345a0b7e 100644
--- a/ash/accessibility/default_accessibility_delegate.cc
+++ b/ash/accessibility/default_accessibility_delegate.cc
@@ -40,4 +40,12 @@ double DefaultAccessibilityDelegate::GetSavedScreenMagnifierScale() {
   return std::numeric_limits<double>::min();
 }
 
+void DefaultAccessibilityDelegate::DispatchAccessibilityEvent(
+    const ui::AXTreeID& tree_id,
+    const std::vector<ui::AXTreeUpdate>& updates,
+    const ui::AXEvent& event) {}
+
+void DefaultAccessibilityDelegate::DispatchTreeDestroyedEvent(
+    const ui::AXTreeID& tree_id) {}
+
 }  // namespace ash
diff --git a/ash/accessibility/default_accessibility_delegate.h b/ash/accessibility/default_accessibility_delegate.h
index 4287df60aeb4dd..2f0914870cc7a3 100644
--- a/ash/accessibility/default_accessibility_delegate.h
+++ b/ash/accessibility/default_accessibility_delegate.h
@@ -11,6 +11,7 @@
 
 namespace ash {
 
+// Used in tests and non-production code like ash_shell_with_content.
 class ASH_EXPORT DefaultAccessibilityDelegate : public AccessibilityDelegate {
  public:
   DefaultAccessibilityDelegate();
@@ -21,6 +22,10 @@ class ASH_EXPORT DefaultAccessibilityDelegate : public AccessibilityDelegate {
   bool ShouldShowAccessibilityMenu() const override;
   void SaveScreenMagnifierScale(double scale) override;
   double GetSavedScreenMagnifierScale() override;
+  void DispatchAccessibilityEvent(const ui::AXTreeID& tree_id,
+                                  const std::vector<ui::AXTreeUpdate>& updates,
+                                  const ui::AXEvent& event) override;
+  void DispatchTreeDestroyedEvent(const ui::AXTreeID& tree_id) override;
 
  private:
   bool screen_magnifier_enabled_ = false;
diff --git a/ash/ash_service.cc b/ash/ash_service.cc
index 5ec00b29f66a91..399948e44b28d8 100644
--- a/ash/ash_service.cc
+++ b/ash/ash_service.cc
@@ -203,6 +203,7 @@ void AshService::CreateService(
     service_manager::mojom::ServiceRequest service,
     const std::string& name,
     service_manager::mojom::PIDReceiverPtr pid_receiver) {
+  // TODO(jamescook): Create the AXHostService here under mash.
   DCHECK_EQ(name, ws::mojom::kServiceName);
   Shell::Get()->window_service_owner()->BindWindowService(std::move(service));
   if (base::FeatureList::IsEnabled(features::kMash)) {
diff --git a/chrome/browser/DEPS b/chrome/browser/DEPS
index 7793cb5d10b610..5ce5910304ca90 100644
--- a/chrome/browser/DEPS
+++ b/chrome/browser/DEPS
@@ -215,11 +215,8 @@ include_rules = [
 
 specific_include_rules = {
   "ash_service_registry\.cc": [
-    # The following are needed for classic mode to create the WindowService.
-    # Once Ash runs out of process no longer needed.
-    "+ash/shell.h",
-
     # Needed for classic mode.
+    "+ash/accessibility/ax_host_service.h",
     "+ash/ash_service.h",
   ],
   # TODO(mash): Fix. https://crbug.com/768439, https://crbug.com/768395.
diff --git a/chrome/browser/ash_service_registry.cc b/chrome/browser/ash_service_registry.cc
index 0a694395be2f95..25d15c5e78e7a3 100644
--- a/chrome/browser/ash_service_registry.cc
+++ b/chrome/browser/ash_service_registry.cc
@@ -4,6 +4,7 @@
 
 #include "chrome/browser/ash_service_registry.h"
 
+#include "ash/accessibility/ax_host_service.h"
 #include "ash/ash_service.h"
 #include "ash/components/quick_launch/public/mojom/constants.mojom.h"
 #include "ash/components/shortcut_viewer/public/mojom/shortcut_viewer.mojom.h"
@@ -15,7 +16,6 @@
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
-#include "chrome/browser/chromeos/accessibility/ax_host_service.h"
 #include "chrome/browser/chromeos/prefs/pref_connector_service.h"
 #include "chrome/grit/generated_resources.h"
 #include "content/public/common/service_manager_connection.h"
@@ -84,20 +84,20 @@ void RegisterInProcessServices(
     (*services)[ash::mojom::kPrefConnectorServiceName] = info;
   }
 
+  if (features::IsMultiProcessMash())
+    return;
+
   {
     // Register the accessibility host service.
     service_manager::EmbeddedServiceInfo info;
     info.task_runner = base::ThreadTaskRunnerHandle::Get();
     info.factory =
         base::BindRepeating([]() -> std::unique_ptr<service_manager::Service> {
-          return std::make_unique<AXHostService>();
+          return std::make_unique<ash::AXHostService>();
         });
     (*services)[ax::mojom::kAXHostServiceName] = info;
   }
 
-  if (features::IsMultiProcessMash())
-    return;
-
   (*services)[ash::mojom::kServiceName] =
       ash::AshService::CreateEmbeddedServiceInfo();
 }
diff --git a/chrome/browser/chromeos/BUILD.gn b/chrome/browser/chromeos/BUILD.gn
index 0a8dc3a98507a5..f26bb5cbb2a2eb 100644
--- a/chrome/browser/chromeos/BUILD.gn
+++ b/chrome/browser/chromeos/BUILD.gn
@@ -263,8 +263,6 @@ source_set("chromeos") {
     "accessibility/accessibility_manager.h",
     "accessibility/accessibility_panel.cc",
     "accessibility/accessibility_panel.h",
-    "accessibility/ax_host_service.cc",
-    "accessibility/ax_host_service.h",
     "accessibility/chromevox_panel.cc",
     "accessibility/chromevox_panel.h",
     "accessibility/dictation_chromeos.cc",
@@ -2019,7 +2017,6 @@ source_set("unit_tests") {
     "../policy/default_geolocation_policy_handler_unittest.cc",
     "../resources/chromeos/zip_archiver/test/char_coding_test.cc",
     "../ui/browser_finder_chromeos_unittest.cc",
-    "accessibility/ax_host_service_unittest.cc",
     "app_mode/startup_app_launcher_unittest.cc",
     "apps/intent_helper/apps_navigation_throttle_unittest.cc",
     "apps/intent_helper/page_transition_util_unittest.cc",
diff --git a/chrome/browser/ui/ash/chrome_accessibility_delegate.cc b/chrome/browser/ui/ash/chrome_accessibility_delegate.cc
index 8001a75e574b2b..10072534a511c8 100644
--- a/chrome/browser/ui/ash/chrome_accessibility_delegate.cc
+++ b/chrome/browser/ui/ash/chrome_accessibility_delegate.cc
@@ -8,6 +8,9 @@
 
 #include "chrome/browser/chromeos/accessibility/accessibility_manager.h"
 #include "chrome/browser/chromeos/accessibility/magnification_manager.h"
+#include "chrome/browser/extensions/api/automation_internal/automation_event_router.h"
+#include "chrome/common/extensions/chrome_extension_messages.h"
+#include "ui/aura/env.h"
 
 using chromeos::AccessibilityManager;
 using chromeos::MagnificationManager;
@@ -42,3 +45,25 @@ double ChromeAccessibilityDelegate::GetSavedScreenMagnifierScale() {
 
   return std::numeric_limits<double>::min();
 }
+
+void ChromeAccessibilityDelegate::DispatchAccessibilityEvent(
+    const ui::AXTreeID& tree_id,
+    const std::vector<ui::AXTreeUpdate>& updates,
+    const ui::AXEvent& event) {
+  ExtensionMsg_AccessibilityEventBundleParams event_bundle;
+  event_bundle.tree_id = tree_id;
+  for (const ui::AXTreeUpdate& update : updates)
+    event_bundle.updates.push_back(update);
+  event_bundle.events.push_back(event);
+  event_bundle.mouse_location = aura::Env::GetInstance()->last_mouse_location();
+
+  // Forward the tree updates and the event to the accessibility extension.
+  extensions::AutomationEventRouter::GetInstance()->DispatchAccessibilityEvents(
+      event_bundle);
+}
+
+void ChromeAccessibilityDelegate::DispatchTreeDestroyedEvent(
+    const ui::AXTreeID& tree_id) {
+  extensions::AutomationEventRouter::GetInstance()->DispatchTreeDestroyedEvent(
+      tree_id, nullptr /* browser_context */);
+}
diff --git a/chrome/browser/ui/ash/chrome_accessibility_delegate.h b/chrome/browser/ui/ash/chrome_accessibility_delegate.h
index 015bd0f2a9b1e8..773acc77d8458f 100644
--- a/chrome/browser/ui/ash/chrome_accessibility_delegate.h
+++ b/chrome/browser/ui/ash/chrome_accessibility_delegate.h
@@ -20,6 +20,10 @@ class ChromeAccessibilityDelegate : public ash::AccessibilityDelegate {
   bool ShouldShowAccessibilityMenu() const override;
   void SaveScreenMagnifierScale(double scale) override;
   double GetSavedScreenMagnifierScale() override;
+  void DispatchAccessibilityEvent(const ui::AXTreeID& tree_id,
+                                  const std::vector<ui::AXTreeUpdate>& updates,
+                                  const ui::AXEvent& event) override;
+  void DispatchTreeDestroyedEvent(const ui::AXTreeID& tree_id) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ChromeAccessibilityDelegate);
diff --git a/chrome/browser/ui/aura/accessibility/DEPS b/chrome/browser/ui/aura/accessibility/DEPS
index b2306413236873..fbced7a81ee084 100644
--- a/chrome/browser/ui/aura/accessibility/DEPS
+++ b/chrome/browser/ui/aura/accessibility/DEPS
@@ -1,6 +1,7 @@
 specific_include_rules = {
   "automation_manager_aura\.cc": [
     # TODO(mash): Fix. https://crbug.com/756054
+    "+ash/accessibility/ax_host_service.h",
     "+ash/shell.h",
     "+ash/wm/window_util.h",
   ],
diff --git a/chrome/browser/ui/aura/accessibility/automation_manager_aura.cc b/chrome/browser/ui/aura/accessibility/automation_manager_aura.cc
index df90b96140a228..abda4616583391 100644
--- a/chrome/browser/ui/aura/accessibility/automation_manager_aura.cc
+++ b/chrome/browser/ui/aura/accessibility/automation_manager_aura.cc
@@ -25,9 +25,9 @@
 #include "ui/views/widget/widget.h"
 
 #if defined(OS_CHROMEOS)
+#include "ash/accessibility/ax_host_service.h"
 #include "ash/shell.h"
 #include "ash/wm/window_util.h"
-#include "chrome/browser/chromeos/accessibility/ax_host_service.h"
 #include "ui/base/ui_base_features.h"
 #include "ui/views/widget/widget_delegate.h"
 #endif
@@ -58,7 +58,8 @@ void AutomationManagerAura::Enable() {
     }
   }
   // Gain access to out-of-process native windows.
-  AXHostService::SetAutomationEnabled(true);
+  // TODO(mash): Split AXHostService into chrome and ash parts.
+  ash::AXHostService::SetAutomationEnabled(true);
 #endif
 }
 
@@ -67,7 +68,7 @@ void AutomationManagerAura::Disable() {
   Reset(true);
 
 #if defined(OS_CHROMEOS)
-  AXHostService::SetAutomationEnabled(false);
+  ash::AXHostService::SetAutomationEnabled(false);
 #endif
 }