diff -urN a/python/mach/mach/mixin/process.py b/python/mach/mach/mixin/process.py
--- a/python/mach/mach/mixin/process.py	2015-07-13 21:48:50.238620500 +0100
+++ b/python/mach/mach/mixin/process.py	2015-07-13 21:59:40.972145000 +0100
@@ -30,7 +30,7 @@
 
 _in_msys = False
 
-if os.environ.get('MSYSTEM', None) == 'MINGW32':
+if os.environ.get('MSYSTEM', None) in ['MINGW32', 'MINGW64', 'MSYS']:
     _in_msys = True
 
     if not _current_shell.lower().endswith('.exe'):