The 'start' event is emitted synchronously downstream of the mocha.run method. Thus, if you are attaching a listener to that event after calling mocha.run (to get the runner instance), you miss the event:

var runner = mocha.run(function (failures) {
  process.on('exit', function () {
    process.exit(failures);  // exit with non-zero status if there were failures
  });
});
runner.on('start', function (Evt) {
  //never called
});

Maybe there is some other way to attach event listeners to the runner prior to calling run, but I haven't found it yet.