--- a/layout/tools/reftest/mach_test_package_commands.py
+++ b/layout/tools/reftest/mach_test_package_commands.py
@@ -83,10 +83,11 @@ class ReftestCommands(object):
 
     def __init__(self, context):
         self.context = context
 
     @Command('reftest', category='testing',
              description='Run the reftest harness.',
              parser=setup_argument_parser)
     def reftest(self, **kwargs):
+        self.context.activate_mozharness_venv()
         kwargs['suite'] = 'reftest'
         return run_reftest(self.context, **kwargs)

--- a/taskcluster/scripts/tester/run-wizard
+++ b/taskcluster/scripts/tester/run-wizard
@@ -53,23 +53,16 @@ def setup():
     build_dir = os.path.expanduser(os.path.join('~', 'workspace', 'build'))
     mach_src = os.path.join(build_dir, 'tests', 'mach')
     mach_dest = os.path.expanduser(os.path.join('~', 'bin', 'mach'))
 
     if os.path.exists(mach_dest):
         os.remove(mach_dest)
     os.symlink(mach_src, mach_dest)
 
-    activate = os.path.join(build_dir, 'venv', 'bin', 'activate')
-    if os.path.isfile(activate):
-        # TODO Support other shells
-        bashrc = os.path.expanduser(os.path.join('~', '.bashrc'))
-        with open(bashrc, 'ab') as f:
-            f.write(". {}".format(activate))
-
     print("""
 Mozharness has finished downloading the build and tests to:
 {}
 
 A limited mach environment has also been set up and added to the $PATH, but
 it may be missing the command you need. To see a list of commands, run:
     $ mach help
 """.lstrip().format(build_dir))

--- a/testing/marionette/mach_test_package_commands.py
+++ b/testing/marionette/mach_test_package_commands.py
@@ -59,9 +59,10 @@ class MachCommands(object):
         self.context = context
 
     @Command(
         'marionette-test', category='testing',
         description='Run a Marionette test (Check UI or the internal JavaScript '
                     'using marionette).',
         parser=setup_marionette_argument_parser)
     def run_marionette_test(self, **kwargs):
+        self.context.activate_mozharness_venv()
         return run_marionette(self.context, **kwargs)

--- a/testing/mochitest/mach_test_package_commands.py
+++ b/testing/mochitest/mach_test_package_commands.py
@@ -88,9 +88,10 @@ class MochitestCommands(object):
 
     def __init__(self, context):
         self.context = context
 
     @Command('mochitest', category='testing',
              description='Run the mochitest harness.',
              parser=setup_argument_parser)
     def mochitest(self, **kwargs):
+        self.context.activate_mozharness_venv()
         return run_mochitest(self.context, **kwargs)

--- a/testing/tools/mach_test_package_bootstrap.py
+++ b/testing/tools/mach_test_package_bootstrap.py
@@ -71,28 +71,57 @@ CATEGORIES = {
         'long': 'The disabled commands are hidden by default. Use -v to display them. '
                 'These commands are unavailable for your current context, '
                 'run "mach <command>" to see why.',
         'priority': 0,
     }
 }
 
 
+IS_WIN = sys.platform in ('win32', 'cygwin')
+
+
 def ancestors(path, depth=0):
     """Emit the parent directories of a path."""
     count = 1
     while path and count != depth:
         yield path
         newpath = os.path.dirname(path)
         if newpath == path:
             break
         path = newpath
         count += 1
 
 
+def activate_mozharness_venv(context):
+    """Activate the mozharness virtualenv in-process."""
+    venv = os.path.join(context.mozharness_workdir,
+                        context.mozharness_config.get('virtualenv_path', 'venv'))
+
+    if not os.path.isdir(venv):
+        print("No mozharness virtualenv detected at '{}'.".format(venv))
+        return 1
+
+    venv_bin = os.path.join(venv, 'Scripts' if IS_WIN else 'bin')
+    activate_path = os.path.join(venv_bin, 'activate_this.py')
+
+    execfile(activate_path, dict(__file__=activate_path))
+
+    if isinstance(os.environ['PATH'], unicode):
+        os.environ['PATH'] = os.environ['PATH'].encode('utf-8')
+
+    # sys.executable is used by mochitest-media to start the websocketprocessbridge,
+    # for some reason it doesn't get set when calling `activate_this.py` so set it
+    # here instead.
+    binary = 'python'
+    if IS_WIN:
+        binary += '.exe'
+    sys.executable = os.path.join(venv_bin, binary)
+
+
 def find_firefox(context):
     """Try to automagically find the firefox binary."""
     import mozinstall
     search_paths = []
 
     # Check for a mozharness setup
     config = context.mozharness_config
     if config and 'binary_path' in config:
@@ -171,16 +200,19 @@ def bootstrap(test_package_root):
                         return json.load(f)
             return {}
 
         if key == 'mozharness_workdir':
             config = context.mozharness_config
             if config:
                 return os.path.join(config['base_work_dir'], config['work_dir'])
 
+        if key == 'activate_mozharness_venv':
+            return types.MethodType(activate_mozharness_venv, context)
+
     mach = mach.main.Mach(os.getcwd())
     mach.populate_context_handler = populate_context
 
     for category, meta in CATEGORIES.items():
         mach.define_category(category, meta['short'], meta['long'],
                              meta['priority'])
 
     for path in MACH_MODULES:

--- a/testing/xpcshell/mach_test_package_commands.py
+++ b/testing/xpcshell/mach_test_package_commands.py
@@ -56,9 +56,10 @@ class MochitestCommands(object):
 
     def __init__(self, context):
         self.context = context
 
     @Command('xpcshell-test', category='testing',
              description='Run the xpcshell harness.',
              parser=parser_desktop)
     def xpcshell(self, **kwargs):
+        self.context.activate_mozharness_venv()
         return run_xpcshell(self.context, **kwargs)