--- a/python/mozbuild/mozbuild/backend/base.py
+++ b/python/mozbuild/mozbuild/backend/base.py
@@ -189,16 +189,25 @@ class BuildBackend(LoggingMixin):
 
         This is the main method used by child classes to react to build
         metadata.
         """
 
     def consume_finished(self):
         """Called when consume() has completed handling all objects."""
 
+    def build(self, config, output, jobs, verbose):
+        """Called when 'mach build' is executed.
+
+        This should return the status value of a subprocess, where 0 denotes
+        success and any other value is an error code. A return value of None
+        indicates that the default 'make -f client.mk' should run.
+        """
+        return None
+
     @contextmanager
     def _write_file(self, path=None, fh=None, mode='rU'):
         """Context manager to write a file.
 
         This is a glorified wrapper around FileAvoidWrite with integration to
         update the summary data on this instance.
 
         Example usage:

--- a/python/mozbuild/mozbuild/mach_commands.py
+++ b/python/mozbuild/mozbuild/mach_commands.py
@@ -31,21 +31,20 @@ from mozbuild.base import (
     MachCommandBase,
     MachCommandConditions as conditions,
     MozbuildObject,
     MozconfigFindException,
     MozconfigLoadException,
     ObjdirMismatchException,
 )
 
-from mozpack.manifests import (
-    InstallManifest,
+from mozbuild.backend import (
+    backends,
+    get_backend_class,
 )
-
-from mozbuild.backend import backends
 from mozbuild.shellutil import quote as shell_quote
 
 
 BUILD_WHAT_HELP = '''
 What to build. Can be a top-level make target or a relative directory. If
 multiple options are provided, they will be built serially. Takes dependency
 information from `topsrcdir/build/dumbmake-dependencies` to build additional
 targets as needed. BUILDING ONLY PARTS OF THE TREE CAN RESULT IN BAD TREE
@@ -408,20 +407,49 @@ class Build(MachCommandBase):
                     status = self._run_make(directory=make_dir, target=make_target,
                         line_handler=output.on_line, log=False, print_directory=False,
                         ensure_exit_code=False, num_jobs=jobs, silent=not verbose,
                         append_env={b'NO_BUILDSTATUS_MESSAGES': b'1'})
 
                     if status != 0:
                         break
             else:
-                status = self._run_make(srcdir=True, filename='client.mk',
-                    line_handler=output.on_line, log=False, print_directory=False,
-                    allow_parallel=False, ensure_exit_code=False, num_jobs=jobs,
-                    silent=not verbose)
+                # Try to call the default backend's build() method. This will
+                # run configure to determine BUILD_BACKENDS if it hasn't run
+                # yet.
+                config = None
+                try:
+                    config = self.config_environment
+                except Exception:
+                    config_rc = self.configure()
+                    if config_rc != 0:
+                        return config_rc
+
+                    # Even if configure runs successfully, we may have trouble
+                    # getting the config_environment for some builds, such as
+                    # OSX Universal builds. These have to go through client.mk
+                    # regardless.
+                    try:
+                        config = self.config_environment
+                    except Exception:
+                        pass
+
+                if config:
+                    active_backend = config.substs.get('BUILD_BACKENDS', [None])[0]
+                    if active_backend:
+                        backend_cls = get_backend_class(active_backend)(config)
+                        status = backend_cls.build(self, output, jobs, verbose)
+
+                # If the backend doesn't specify a build() method, then just
+                # call client.mk directly.
+                if status is None:
+                    status = self._run_make(srcdir=True, filename='client.mk',
+                        line_handler=output.on_line, log=False, print_directory=False,
+                        allow_parallel=False, ensure_exit_code=False, num_jobs=jobs,
+                        silent=not verbose)
 
                 self.log(logging.WARNING, 'warning_summary',
                     {'count': len(monitor.warnings_database)},
                     '{count} compiler warnings present.')
 
             monitor.finish(record_usage=status==0)
 
         high_finder, finder_percent = monitor.have_high_finder_usage()