--- a/python/mach/mach/main.py
+++ b/python/mach/mach/main.py
@@ -325,16 +325,18 @@ To see more help for a specific command,
         orig_stdin = sys.stdin
         orig_stdout = sys.stdout
         orig_stderr = sys.stderr
 
         sys.stdin = stdin
         sys.stdout = stdout
         sys.stderr = stderr
 
+        orig_env = dict(os.environ)
+
         try:
             if stdin.encoding is None:
                 sys.stdin = codecs.getreader('utf-8')(stdin)
 
             if stdout.encoding is None:
                 sys.stdout = codecs.getwriter('utf-8')(stdout)
 
             if stderr.encoding is None:
@@ -357,16 +359,19 @@ To see more help for a specific command,
             exc_type, exc_value, exc_tb = sys.exc_info()
             stack = traceback.extract_tb(exc_tb)
 
             self._print_exception(sys.stdout, exc_type, exc_value, stack)
 
             return 1
 
         finally:
+            os.environ.clear()
+            os.environ.update(orig_env)
+
             sys.stdin = orig_stdin
             sys.stdout = orig_stdout
             sys.stderr = orig_stderr
 
     def _run(self, argv):
         # Load settings as early as possible so things in dispatcher.py
         # can use them.
         for provider in Registrar.settings_providers: