--- a/client.mk
+++ b/client.mk
@@ -44,18 +44,18 @@ define CR
 
 endef
 
 # As $(shell) doesn't preserve newlines, use sed to replace them with an
 # unlikely sequence (||), which is then replaced back to newlines by make
 # before evaluation. $(shell) replacing newlines with spaces, || is always
 # followed by a space (since sed doesn't remove newlines), except on the
 # last line, so replace both '|| ' and '||'.
-MOZCONFIG_CONTENT := $(subst ||,$(CR),$(subst || ,$(CR),$(shell $(TOPSRCDIR)/mach environment --format=client.mk | sed 's/$$/||/')))
-$(eval $(MOZCONFIG_CONTENT))
+MOZCONFIG_CONTENT := $(subst ||,$(CR),$(subst || ,$(CR),$(shell cat $(OBJDIR)/.mozconfig-client-mk | sed 's/$$/||/')))
+include $(OBJDIR)/.mozconfig-client-mk
 
 export FOUND_MOZCONFIG
 
 # As '||' was used as a newline separator, it means it's not occurring in
 # lines themselves. It can thus safely be used to replaces normal spaces,
 # to then replace newlines with normal spaces. This allows to get a list
 # of mozconfig output lines.
 MOZCONFIG_OUT_LINES := $(subst $(CR), ,$(subst $(NULL) $(NULL),||,$(MOZCONFIG_CONTENT)))
@@ -103,18 +103,17 @@ ifndef MACH
 $(error client.mk must be used via `mach`. Try running \
 `./mach $(firstword $(MAKECMDGOALS) $(.DEFAULT_GOAL))`)
 endif
 
 # Define mkdir
 include $(TOPSRCDIR)/config/makefiles/makeutils.mk
 include $(TOPSRCDIR)/config/makefiles/autotargets.mk
 
-# For now, only output "export" lines and lines containing UPLOAD_EXTRA_FILES
-# from mach environment --format=client.mk output.
+# For now, only output "export" lines and lines containing UPLOAD_EXTRA_FILES.
 MOZCONFIG_MK_LINES := $(filter export||% UPLOAD_EXTRA_FILES% %UPLOAD_EXTRA_FILES%,$(MOZCONFIG_OUT_LINES))
 $(OBJDIR)/.mozconfig.mk: $(TOPSRCDIR)/client.mk $(FOUND_MOZCONFIG) $(call mkdir_deps,$(OBJDIR)) $(OBJDIR)/CLOBBER
 	$(if $(MOZCONFIG_MK_LINES),( $(foreach line,$(MOZCONFIG_MK_LINES), echo '$(subst ||, ,$(line))';) )) > $@
 
 # Include that makefile so that it is created. This should not actually change
 # the environment since MOZCONFIG_CONTENT, which MOZCONFIG_OUT_LINES derives
 # from, has already been eval'ed.
 include $(OBJDIR)/.mozconfig.mk

--- a/python/mozbuild/mozbuild/controller/building.py
+++ b/python/mozbuild/mozbuild/controller/building.py
@@ -45,16 +45,17 @@ from ..testing import (
 from ..compilation.warnings import (
     WarningsCollector,
     WarningsDatabase,
 )
 from ..shellutil import (
     quote as shell_quote,
 )
 from ..util import (
+    FileAvoidWrite,
     mkdir,
     resolve_target_to_make,
 )
 
 
 FINDER_SLOW_MESSAGE = '''
 ===================
 PERFORMANCE WARNING
@@ -1318,16 +1319,34 @@ class BuildDriver(MozbuildObject):
 
     def _run_client_mk(self, target=None, line_handler=None, jobs=0,
                        verbose=None, keep_going=False, append_env=None):
         append_env = dict(append_env or {})
         append_env['TOPSRCDIR'] = self.topsrcdir
 
         append_env['CONFIG_GUESS'] = self.resolve_config_guess()
 
+        mozconfig = self.mozconfig
+        mozconfig_client_mk = os.path.join(self.topobjdir,
+                                           '.mozconfig-client-mk')
+        with FileAvoidWrite(mozconfig_client_mk) as fh:
+            for arg in mozconfig['make_extra'] or []:
+                fh.write(arg)
+                fh.write(b'\n')
+            if mozconfig['make_flags']:
+                fh.write(b'MOZ_MAKE_FLAGS=%s\n' % b' '.join(mozconfig['make_flags']))
+            objdir = mozpath.normsep(self.topobjdir)
+            fh.write(b'MOZ_OBJDIR=%s\n' % objdir)
+            fh.write(b'OBJDIR=%s\n' % objdir)
+            if mozconfig['path']:
+                fh.write(b'FOUND_MOZCONFIG=%s\n' %
+                         mozpath.normsep(mozconfig['path']))
+
+        append_env['OBJDIR'] = mozpath.normsep(self.topobjdir)
+
         return self._run_make(srcdir=True,
                               filename='client.mk',
                               allow_parallel=False,
                               ensure_exit_code=False,
                               print_directory=False,
                               target=target,
                               line_handler=line_handler,
                               log=False,

--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -1016,17 +1016,17 @@ class Makefiles(MachCommandBase):
                 if f == 'Makefile.in':
                     yield os.path.join(root, f)
 
 @CommandProvider
 class MachDebug(MachCommandBase):
     @Command('environment', category='build-dev',
         description='Show info about the mach and build environment.')
     @CommandArgument('--format', default='pretty',
-        choices=['pretty', 'client.mk', 'configure', 'json'],
+        choices=['pretty', 'configure', 'json'],
         help='Print data in the given format.')
     @CommandArgument('--output', '-o', type=str,
         help='Output to the given file.')
     @CommandArgument('--verbose', '-v', action='store_true',
         help='Print verbose output.')
     def environment(self, format, output=None, verbose=False):
         func = getattr(self, '_environment_%s' % format.replace('.', '_'))
 
@@ -1084,29 +1084,16 @@ class MachDebug(MachCommandBase):
                 print('config substitutions:', file=out)
                 for k in sorted(config.substs):
                     print('\t%s: %s' % (k, config.substs[k]), file=out)
 
                 print('config defines:', file=out)
                 for k in sorted(config.defines):
                     print('\t%s' % k, file=out)
 
-    def _environment_client_mk(self, out, verbose):
-        if self.mozconfig['make_extra']:
-            for arg in self.mozconfig['make_extra']:
-                print(arg, file=out)
-        if self.mozconfig['make_flags']:
-            print('MOZ_MAKE_FLAGS=%s' % ' '.join(self.mozconfig['make_flags']))
-        objdir = mozpath.normsep(self.topobjdir)
-        print('MOZ_OBJDIR=%s' % objdir, file=out)
-        print('OBJDIR=%s' % objdir, file=out)
-        if self.mozconfig['path']:
-            print('FOUND_MOZCONFIG=%s' % mozpath.normsep(self.mozconfig['path']),
-                file=out)
-
     def _environment_json(self, out, verbose):
         import json
         class EnvironmentEncoder(json.JSONEncoder):
             def default(self, obj):
                 if isinstance(obj, MozbuildObject):
                     result = {
                         'topsrcdir': obj.topsrcdir,
                         'topobjdir': obj.topobjdir,