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
@@ -23,7 +23,8 @@
     parser = argparse.ArgumentParser()
     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('--env', type=str, nargs='*', required=False, default=[])
+    parser.add_argument('--prepend_env', type=str, nargs='*', required=False, default=[])
     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().
diff --git a/libcxx/utils/ssh.py b/libcxx/utils/ssh.py
--- a/libcxx/utils/ssh.py
+++ b/libcxx/utils/ssh.py
@@ -47,7 +47,8 @@
     parser.add_argument('--extra-ssh-args', type=str, required=False)
     parser.add_argument('--extra-scp-args', type=str, required=False)
     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('--env', type=str, nargs='*', required=False, default=[])
+    parser.add_argument('--prepend_env', type=str, nargs='*', required=False, default=[])
     parser.add_argument("command", nargs=argparse.ONE_OR_MORE)
     args = parser.parse_args()
     commandLine = args.command
@@ -108,6 +109,12 @@
         # temporary directory on the remote host.
         commandLine = (pathOnRemote(x) if isTestExe(x) else x for x in commandLine)
         remoteCommands.append('cd {}'.format(tmp))
+
+        if args.prepend_env:
+            # We can't sensibly know the original value of the env vars
+            # in order to prepend to them, so just overwrite these variables.
+            args.env.extend(args.prepend_env)
+
         if args.env:
             env = list(map(cmd_quote, args.env))
             remoteCommands.append('export {}'.format(' '.join(args.env)))
diff --git a/libcxxabi/test/configs/llvm-libc++abi-mingw.cfg.in b/libcxxabi/test/configs/llvm-libc++abi-mingw.cfg.in
--- a/libcxxabi/test/configs/llvm-libc++abi-mingw.cfg.in
+++ b/libcxxabi/test/configs/llvm-libc++abi-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/libcxxabi/test/configs/llvm-libc++abi-shared-clangcl.cfg.in b/libcxxabi/test/configs/llvm-libc++abi-shared-clangcl.cfg.in
--- a/libcxxabi/test/configs/llvm-libc++abi-shared-clangcl.cfg.in
+++ b/libcxxabi/test/configs/llvm-libc++abi-shared-clangcl.cfg.in
@@ -11,7 +11,7 @@
     '-nostdlib -L %{lib} -lc++ -lc++abi -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/libcxxabi/test/configs/llvm-libc++abi-static-clangcl.cfg.in b/libcxxabi/test/configs/llvm-libc++abi-static-clangcl.cfg.in
--- a/libcxxabi/test/configs/llvm-libc++abi-static-clangcl.cfg.in
+++ b/libcxxabi/test/configs/llvm-libc++abi-static-clangcl.cfg.in
@@ -11,7 +11,7 @@
     '-nostdlib -L %{lib} -llibc++ -llibc++abi -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/libunwind/test/configs/llvm-libunwind-mingw.cfg.in b/libunwind/test/configs/llvm-libunwind-mingw.cfg.in
--- a/libunwind/test/configs/llvm-libunwind-mingw.cfg.in
+++ b/libunwind/test/configs/llvm-libunwind-mingw.cfg.in
@@ -11,7 +11,7 @@
     '-L %{lib} -lunwind'
 ))
 config.substitutions.append(('%{exec}',
-    '%{executor} --execdir %T --env PATH=%{lib} -- '
+    '%{executor} --execdir %T --prepend_env PATH=%{lib} -- '
 ))
 
 import os, site