diff --git a/taskcluster/ci/build/kind.yml b/taskcluster/ci/build/kind.yml
index 53cc565d056e..c87d17428d0e 100644
--- a/taskcluster/ci/build/kind.yml
+++ b/taskcluster/ci/build/kind.yml
@@ -33,6 +33,7 @@ job-defaults:
     worker:
         env:
             MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE: system
+    use-system-python: false
     run:
         mozconfig-variant:
             by-release-type:
diff --git a/taskcluster/gecko_taskgraph/transforms/job/__init__.py b/taskcluster/gecko_taskgraph/transforms/job/__init__.py
index 626fe1abc916..9b6924b6053b 100644
--- a/taskcluster/gecko_taskgraph/transforms/job/__init__.py
+++ b/taskcluster/gecko_taskgraph/transforms/job/__init__.py
@@ -62,6 +62,7 @@ job_description_schema = Schema(
             "optimization"
         ],
         Optional("use-sccache"): task_description_schema["use-sccache"],
+        Optional("use-system-python"): bool,
         Optional("priority"): task_description_schema["priority"],
         # The "when" section contains descriptions of the circumstances under which
         # this task should be included in the task graph.  This will be converted
@@ -241,6 +242,34 @@ def get_attribute(dict, key, attributes, attribute_name):
         dict[key] = value
 
 
+@transforms.add
+def use_system_python(config, jobs):
+    for job in jobs:
+        if job.pop("use-system-python", True):
+            yield job
+        else:
+            fetches = job.setdefault("fetches", {})
+            toolchain = fetches.setdefault("toolchain", [])
+
+            moz_python_home = mozpath.join("fetches", "python")
+            if "win" in job["worker"]["os"]:
+                platform = "win64"
+            elif "linux" in job["worker"]["os"]:
+                platform = "linux64"
+            elif "macosx" in job["worker"]["os"]:
+                platform = "macosx64"
+            else:
+                raise ValueError("unexpected worker.os value {}".format(platform))
+
+            toolchain.append("{}-python".format(platform))
+
+            worker = job.setdefault("worker", {})
+            env = worker.setdefault("env", {})
+            env["MOZ_PYTHON_HOME"] = moz_python_home
+
+            yield job
+
+
 @transforms.add
 def use_fetches(config, jobs):
     artifact_names = {}
diff --git a/taskcluster/gecko_taskgraph/transforms/job/mozharness.py b/taskcluster/gecko_taskgraph/transforms/job/mozharness.py
index bbcb18d55c73..c023562437a8 100644
--- a/taskcluster/gecko_taskgraph/transforms/job/mozharness.py
+++ b/taskcluster/gecko_taskgraph/transforms/job/mozharness.py
@@ -283,11 +283,25 @@ def mozharness_on_generic_worker(config, job, taskdesc):
 
     mh_command = []
     if job["worker"]["os"] == "windows":
-        mh_command.append("c:/mozilla-build/python3/python3.exe")
+        system_python_dir = "c:/mozilla-build/python3/"
         gecko_path = "%GECKO_PATH%"
     else:
+        system_python_dir = ""
         gecko_path = "$GECKO_PATH"
 
+    if run.get("use-system-python", False):
+        python_bindir = system_python_dir
+    else:
+        # $MOZ_PYTHON_HOME is going to be substituted in run-task, when we
+        # know the actual MOZ_PYTHON_HOME value.
+        is_windows = job["worker"]["os"] == "windows"
+        if is_windows:
+            python_bindir = "%MOZ_PYTHON_HOME%/"
+        else:
+            python_bindir = "${MOZ_PYTHON_HOME}/bin/"
+
+    mh_command = ["{}python3".format(python_bindir)]
+
     mh_command += [
         f"{gecko_path}/mach",
         "python",
diff --git a/taskcluster/scripts/run-task b/taskcluster/scripts/run-task
index 9548a81bab80..fb0d32f8f52d 100755
--- a/taskcluster/scripts/run-task
+++ b/taskcluster/scripts/run-task
@@ -27,7 +27,6 @@ import errno
 import io
 import json
 import os
-import platform
 import random
 import re
 import shutil
@@ -936,7 +935,7 @@ def main(args):
     resource_process = None
 
     try:
-        for k in ('GECKO_PATH', 'MOZ_FETCHES_DIR', 'UPLOAD_DIR'):
+        for k in ('GECKO_PATH', 'MOZ_FETCHES_DIR', 'UPLOAD_DIR', 'MOZ_PYTHON_HOME'):
             if k in os.environ:
                 # Normalize paths to use forward slashes. Some shell scripts
                 # tolerate that better on Windows.
@@ -948,6 +947,43 @@ def main(args):
         if 'MOZ_FETCHES' in os.environ:
             fetch_artifacts()
 
+            # If Python is a fetch dependency, add it to the PATH and setting
+            # the mozilla-specific MOZ_PYTHON_HOME to relocate binaries.
+            if 'MOZ_PYTHON_HOME' in os.environ:
+
+                print_line(b'setup',
+                           b'Setting up local python environment\n')
+                prev = [os.environ['PATH']] if 'PATH' in os.environ else []
+
+                moz_python_home = os.environ['MOZ_PYTHON_HOME']
+                if IS_WINDOWS:
+                    ext = '.exe'
+                    moz_python_bindir = moz_python_home
+                else:
+                    ext = ''
+                    moz_python_bindir = moz_python_home + '/bin'
+
+
+                # just a sanity check
+                candidate = os.path.join(moz_python_bindir, f'python3{ext}')
+                if not os.path.exists(candidate):
+                    raise RuntimeError("Inconsistent Python installation: "
+                                       "archive found, but no python3 binary "
+                                       "detected")
+
+                new = os.environ['PATH'] = os.pathsep.join([moz_python_bindir]
+                                                           + prev)
+
+                # Relocate the python binary. Standard way uses PYTHONHOME, but
+                # this conflicts with system python (e.g. used by hg) so we
+                # maintain a small patch to use MOZPYTHONHOME instead.
+                os.environ['MOZPYTHONHOME'] = moz_python_home
+
+                print_line(b'setup',
+                           b'updated PATH with python artifact: '
+                           + new.encode() + b'\n')
+
+
         resource_process = maybe_run_resource_monitoring()
 
         return run_and_prefix_output(b'task', task_args, cwd=args.task_cwd)