diff --git a/taskcluster/scripts/builder/build-linux.sh b/taskcluster/scripts/builder/build-linux.sh
index 64224dfcd33a..8b646da52221 100755
--- a/taskcluster/scripts/builder/build-linux.sh
+++ b/taskcluster/scripts/builder/build-linux.sh
@@ -88,7 +88,7 @@ export TOOLTOOL_CACHE
 
 config_path_cmds=""
 for path in ${MOZHARNESS_CONFIG_PATHS}; do
-    config_path_cmds="${config_path_cmds} --extra-config-path ${WORKSPACE}/build/src/${path}"
+    config_path_cmds="${config_path_cmds} --extra-config-path ${GECKO_PATH}/${path}"
 done
 
 # support multiple, space delimited, config files
@@ -117,7 +117,7 @@ fi
 
 cd /builds/worker
 
-python2.7 $WORKSPACE/build/src/testing/${MOZHARNESS_SCRIPT} \
+$GECKO_PATH/mach python $GECKO_PATH/testing/${MOZHARNESS_SCRIPT} \
   ${config_path_cmds} \
   ${config_cmds} \
   $debug_flag \
diff --git a/taskcluster/scripts/tester/test-linux.sh b/taskcluster/scripts/tester/test-linux.sh
index 4823e3f185ff..3c5d36889d8e 100755
--- a/taskcluster/scripts/tester/test-linux.sh
+++ b/taskcluster/scripts/tester/test-linux.sh
@@ -19,6 +19,7 @@ fi
 
 # Inputs, with defaults
 
+: GECKO_PATH                    ${GECKO_PATH}
 : MOZHARNESS_PATH               ${MOZHARNESS_PATH}
 : MOZHARNESS_URL                ${MOZHARNESS_URL}
 : MOZHARNESS_SCRIPT             ${MOZHARNESS_SCRIPT}
@@ -183,16 +184,20 @@ if [ -n "$MOZHARNESS_OPTIONS" ]; then
     done
 fi
 
+# Use |mach python| if a source checkout exists so in-tree packages are
+# available.
+[[ -d "${GECKO_PATH}" ]] && python="${GECKO_PATH}/mach python" || python="python2.7"
+
+# Save the computed mozharness command to a binary which is useful for
+# interactive mode.
 mozharness_bin="$HOME/bin/run-mozharness"
 mkdir -p $(dirname $mozharness_bin)
 
-# Save the computed mozharness command to a binary which is useful
-# for interactive mode.
 echo -e "#!/usr/bin/env bash
 # Some mozharness scripts assume base_work_dir is in
 # the current working directory, see bug 1279237
 cd "$WORKSPACE"
-cmd=\"python2.7 ${MOZHARNESS_PATH}/scripts/${MOZHARNESS_SCRIPT} ${config_cmds} ${options} ${@} \${@}\"
+cmd=\"${python} ${MOZHARNESS_PATH}/scripts/${MOZHARNESS_SCRIPT} ${config_cmds} ${options} ${@} \${@}\"
 echo \"Running: \${cmd}\"
 exec \${cmd}" > ${mozharness_bin}
 chmod +x ${mozharness_bin}
diff --git a/taskcluster/taskgraph/transforms/job/mozharness.py b/taskcluster/taskgraph/transforms/job/mozharness.py
index 7e03932a0002..320a00daf0d5 100644
--- a/taskcluster/taskgraph/transforms/job/mozharness.py
+++ b/taskcluster/taskgraph/transforms/job/mozharness.py
@@ -300,16 +300,19 @@ def mozharness_on_generic_worker(config, job, taskdesc):
             "Task generation for mozharness build jobs currently only supported on Windows"
         )
 
-    mh_command = [r'c:\mozilla-build\python\python.exe']
-    mh_command.append('\\'.join([r'.\build\src\testing', run['script'].replace('/', '\\')]))
+    gecko = env['GECKO_PATH'].replace('.', '%cd%')
+    mh_command = [
+            'c:/mozilla-build/python/python.exe',
+            '{}/mach'.format(gecko), 'python',
+    ]
+    mh_command.append('/'.join([gecko, 'testing', run['script']]))
 
     if 'config-paths' in run:
         for path in run['config-paths']:
-            mh_command.append(r'--extra-config-path '
-                              r'.\build\src\{}'.format(path.replace('/', '\\')))
+            mh_command.append('--extra-config-path {}/{}'.format(gecko, path))
 
     for cfg in run['config']:
-        mh_command.append('--config ' + cfg.replace('/', '\\'))
+        mh_command.append('--config ' + cfg)
     if run['use-magic-mh-args']:
         mh_command.append('--branch ' + config.params['project'])
     mh_command.append(r'--work-dir %cd:Z:=z:%\build')