--- a/python/mach/mach/mixin/process.py
+++ b/python/mach/mach/mixin/process.py
@@ -25,17 +25,17 @@ elif 'MOZILLABUILD' in os.environ:
     _current_shell = os.environ['MOZILLABUILD'] + '/msys/bin/sh.exe'
 elif 'COMSPEC' in os.environ:
     _current_shell = os.environ['COMSPEC']
 else:
     raise Exception('Could not detect environment shell!')
 
 _in_msys = False
 
-if os.environ.get('MSYSTEM', None) == 'MINGW32':
+if os.environ.get('MSYSTEM', None) in ('MINGW32', 'MINGW64'):
     _in_msys = True
 
     if not _current_shell.lower().endswith('.exe'):
         _current_shell += '.exe'
 
 
 class ProcessExecutionMixin(LoggingMixin):
     """Mix-in that provides process execution functionality."""