diff --git a/libcxx/test/configs/llvm-libc++-mingw.cfg.in b/libcxx/test/configs/llvm-libc++-mingw.cfg.in
--- a/libcxx/test/configs/llvm-libc++-mingw.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-mingw.cfg.in
@@ -11,7 +11,7 @@
     '-nostdlib++ -L %{lib} -lc++'
 ))
 config.substitutions.append(('%{exec}',
-    '%{executor} --execdir %T --env PATH=%{lib} -- '
+    '%{executor} --execdir %T --prepend_env PATH=%{lib} -- '
 ))
 
 import os, site
diff --git a/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in b/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
--- a/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-shared-clangcl.cfg.in
@@ -11,7 +11,7 @@
     '-nostdlib -L %{lib} -lc++ -lmsvcrt -lmsvcprt -loldnames'
 ))
 config.substitutions.append(('%{exec}',
-    '%{executor} --execdir %T --env PATH=%{lib} -- '
+    '%{executor} --execdir %T --prepend_env PATH=%{lib} -- '
 ))
 
 import os, site
diff --git a/libcxx/test/configs/llvm-libc++-shared-no-vcruntime-clangcl.cfg.in b/libcxx/test/configs/llvm-libc++-shared-no-vcruntime-clangcl.cfg.in
--- a/libcxx/test/configs/llvm-libc++-shared-no-vcruntime-clangcl.cfg.in
+++ b/libcxx/test/configs/llvm-libc++-shared-no-vcruntime-clangcl.cfg.in
@@ -12,7 +12,7 @@
     '-nostdlib -L %{lib} -lc++ -lmsvcrt -lmsvcprt -loldnames'
 ))
 config.substitutions.append(('%{exec}',
-    '%{executor} --execdir %T --env PATH=%{lib} -- '
+    '%{executor} --execdir %T --prepend_env PATH=%{lib} -- '
 ))
 
 import os, site
diff --git a/libcxx/utils/run.py b/libcxx/utils/run.py
--- a/libcxx/utils/run.py
+++ b/libcxx/utils/run.py
@@ -24,6 +24,7 @@
     parser.add_argument('--execdir', type=str, required=True)
     parser.add_argument('--codesign_identity', type=str, required=False, default=None)
     parser.add_argument('--env', type=str, nargs='*', required=False, default=dict())
+    parser.add_argument('--prepend_env', type=str, nargs='*', required=False, default=dict())
     parser.add_argument("command", nargs=argparse.ONE_OR_MORE)
     args = parser.parse_args()
     commandLine = args.command
@@ -43,6 +44,14 @@
 
     # Extract environment variables into a dictionary
     env = {k : v  for (k, v) in map(lambda s: s.split('=', 1), args.env)}
+
+    # Set environment variables where we prepend the given value to the
+    # existing environment variable.
+    for (k, v) in map(lambda s: s.split('=', 1), args.prepend_env):
+        if k in os.environ:
+            v = v + os.pathsep + os.environ[k]
+        env[k] = v
+
     if platform.system() == 'Windows':
         # Pass some extra variables through on Windows:
         # COMSPEC is needed for running subprocesses via std::system().