diff --git a/clang-tools-extra/clangd/ConfigCompile.cpp b/clang-tools-extra/clangd/ConfigCompile.cpp
--- a/clang-tools-extra/clangd/ConfigCompile.cpp
+++ b/clang-tools-extra/clangd/ConfigCompile.cpp
@@ -31,6 +31,7 @@
 #include "Features.inc"
 #include "TidyProvider.h"
 #include "support/Logger.h"
+#include "support/Path.h"
 #include "support/Trace.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
@@ -101,9 +102,11 @@
   // Normalized Fragment::SourceInfo::Directory.
   std::string FragmentDirectory;
 
-  llvm::Optional<llvm::Regex> compileRegex(const Located<std::string> &Text) {
+  llvm::Optional<llvm::Regex>
+  compileRegex(const Located<std::string> &Text,
+               llvm::Regex::RegexFlags Flags = llvm::Regex::NoFlags) {
     std::string Anchored = "^(" + *Text + ")$";
-    llvm::Regex Result(Anchored);
+    llvm::Regex Result(Anchored, Flags);
     std::string RegexError;
     if (!Result.isValid(RegexError)) {
       diag(Error, "Invalid regex " + Anchored + ": " + RegexError, Text.Range);
@@ -195,9 +198,15 @@
     if (F.HasUnrecognizedCondition)
       Out.Conditions.push_back([&](const Params &) { return false; });
 
+#ifdef CLANGD_PATH_CASE_INSENSITIVE
+    llvm::Regex::RegexFlags Flags = llvm::Regex::IgnoreCase;
+#else
+    llvm::Regex::RegexFlags Flags = llvm::Regex::NoFlags;
+#endif
+
     auto PathMatch = std::make_unique<std::vector<llvm::Regex>>();
     for (auto &Entry : F.PathMatch) {
-      if (auto RE = compileRegex(Entry))
+      if (auto RE = compileRegex(Entry, Flags))
         PathMatch->push_back(std::move(*RE));
     }
     if (!PathMatch->empty()) {
@@ -218,7 +227,7 @@
 
     auto PathExclude = std::make_unique<std::vector<llvm::Regex>>();
     for (auto &Entry : F.PathExclude) {
-      if (auto RE = compileRegex(Entry))
+      if (auto RE = compileRegex(Entry, Flags))
         PathExclude->push_back(std::move(*RE));
     }
     if (!PathExclude->empty()) {
@@ -349,7 +358,8 @@
       return;
     Spec.MountPoint = std::move(*AbsPath);
     Out.Apply.push_back([Spec(std::move(Spec))](const Params &P, Config &C) {
-      if (!P.Path.startswith(Spec.MountPoint))
+      if (!pathIsAncestor(Spec.MountPoint, P.Path,
+                          llvm::sys::path::Style::posix))
         return;
       C.Index.External = Spec;
       // Disable background indexing for the files under the mountpoint.
diff --git a/clang-tools-extra/clangd/support/Path.h b/clang-tools-extra/clangd/support/Path.h
--- a/clang-tools-extra/clangd/support/Path.h
+++ b/clang-tools-extra/clangd/support/Path.h
@@ -10,8 +10,14 @@
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_SUPPORT_PATH_H
 
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Path.h"
 #include <string>
 
+/// Whether current platform treats paths case insensitively.
+#if defined(_WIN32) || defined(__APPLE__)
+#define CLANGD_PATH_CASE_INSENSITIVE
+#endif
+
 namespace clang {
 namespace clangd {
 
@@ -28,6 +34,10 @@
 std::string maybeCaseFoldPath(PathRef Path);
 bool pathEqual(PathRef, PathRef);
 
+/// Checks if \p Ancestor is a proper ancestor of \p Path.
+bool pathIsAncestor(
+    PathRef Ancestor, PathRef Path,
+    llvm::sys::path::Style Style = llvm::sys::path::Style::native);
 } // namespace clangd
 } // namespace clang
 
diff --git a/clang-tools-extra/clangd/support/Path.cpp b/clang-tools-extra/clangd/support/Path.cpp
--- a/clang-tools-extra/clangd/support/Path.cpp
+++ b/clang-tools-extra/clangd/support/Path.cpp
@@ -7,24 +7,31 @@
 //===----------------------------------------------------------------------===//
 
 #include "support/Path.h"
+#include "llvm/Support/Path.h"
 namespace clang {
 namespace clangd {
 
-std::string maybeCaseFoldPath(PathRef Path) {
-#if defined(_WIN32) || defined(__APPLE__)
-  return Path.lower();
-#else
-  return std::string(Path);
-#endif
-}
+#ifdef CLANGD_PATH_CASE_INSENSITIVE
+std::string maybeCaseFoldPath(PathRef Path) { return Path.lower(); }
+bool pathEqual(PathRef A, PathRef B) { return A.equals_lower(B); }
+#else  // NOT CLANGD_PATH_CASE_INSENSITIVE
+std::string maybeCaseFoldPath(PathRef Path) { return Path.str(); }
+bool pathEqual(PathRef A, PathRef B) { return A == B; }
+#endif // CLANGD_PATH_CASE_INSENSITIVE
 
-bool pathEqual(PathRef A, PathRef B) {
-#if defined(_WIN32) || defined(__APPLE__)
-  return A.equals_lower(B);
-#else
-  return A == B;
-#endif
+bool pathIsAncestor(PathRef Ancestor, PathRef Path,
+                    llvm::sys::path::Style Style) {
+  // If ancestor ends with a separator drop that, so that we can match /foo/ as
+  // a parent of /foo.
+  if (llvm::sys::path::is_separator(Ancestor.back(), Style))
+    Ancestor = Ancestor.drop_back();
+  // Ensure Path starts with Ancestor.
+  if (!pathEqual(Ancestor, Path.take_front(Ancestor.size())))
+    return false;
+  Path = Path.drop_front(Ancestor.size());
+  // Then make sure either two paths are equal or Path has a separator
+  // afterwards.
+  return Path.empty() || llvm::sys::path::is_separator(Path.front(), Style);
 }
-
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/clangd/unittests/CMakeLists.txt b/clang-tools-extra/clangd/unittests/CMakeLists.txt
--- a/clang-tools-extra/clangd/unittests/CMakeLists.txt
+++ b/clang-tools-extra/clangd/unittests/CMakeLists.txt
@@ -104,6 +104,7 @@
   support/FunctionTests.cpp
   support/MarkupTests.cpp
   support/MemoryTreeTests.cpp
+  support/PathTests.cpp
   support/ThreadingTests.cpp
   support/TestTracer.cpp
   support/TraceTests.cpp
diff --git a/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
--- a/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ConfigCompileTests.cpp
@@ -99,6 +99,25 @@
   Frag.If.PathMatch.emplace_back("ba*r");
   EXPECT_FALSE(compileAndApply());
   EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+
+  // Only matches case-insensitively.
+  Frag = {};
+  Frag.If.PathMatch.emplace_back("B*R");
+  EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+#ifdef CLANGD_PATH_CASE_INSENSITIVE
+  EXPECT_TRUE(compileAndApply());
+#else
+  EXPECT_FALSE(compileAndApply());
+#endif
+
+  Frag = {};
+  Frag.If.PathExclude.emplace_back("B*R");
+  EXPECT_THAT(Diags.Diagnostics, IsEmpty());
+#ifdef CLANGD_PATH_CASE_INSENSITIVE
+  EXPECT_FALSE(compileAndApply());
+#else
+  EXPECT_TRUE(compileAndApply());
+#endif
 }
 
 TEST_F(ConfigCompileTests, CompileCommands) {
@@ -406,6 +425,22 @@
   ASSERT_THAT(Diags.Diagnostics, IsEmpty());
   ASSERT_TRUE(Conf.Index.External);
   EXPECT_THAT(Conf.Index.External->MountPoint, FooPath);
+
+  // Only matches case-insensitively.
+  BazPath = testPath("fOo/baz.h", llvm::sys::path::Style::posix);
+  BazPath = llvm::sys::path::convert_to_slash(BazPath);
+  Parm.Path = BazPath;
+
+  FooPath = testPath("FOO/", llvm::sys::path::Style::posix);
+  Frag = GetFrag("", FooPath.c_str());
+  compileAndApply();
+  ASSERT_THAT(Diags.Diagnostics, IsEmpty());
+#ifdef CLANGD_PATH_CASE_INSENSITIVE
+  ASSERT_TRUE(Conf.Index.External);
+  EXPECT_THAT(Conf.Index.External->MountPoint, FooPath);
+#else
+  ASSERT_FALSE(Conf.Index.External);
+#endif
 }
 } // namespace
 } // namespace config
diff --git a/clang-tools-extra/clangd/unittests/RenameTests.cpp b/clang-tools-extra/clangd/unittests/RenameTests.cpp
--- a/clang-tools-extra/clangd/unittests/RenameTests.cpp
+++ b/clang-tools-extra/clangd/unittests/RenameTests.cpp
@@ -1125,7 +1125,7 @@
   EXPECT_THAT(Results.GlobalChanges.keys(),
               UnorderedElementsAre(Main, testPath("other.cc")));
 
-#if defined(_WIN32) || defined(__APPLE__)
+#ifdef CLANGD_PATH_CASE_INSENSITIVE
   // On case-insensitive systems, no duplicates if AST vs index case differs.
   // https://github.com/clangd/clangd/issues/665
   TU.Filename = "MAIN.CC";
diff --git a/clang-tools-extra/clangd/unittests/support/PathTests.cpp b/clang-tools-extra/clangd/unittests/support/PathTests.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/unittests/support/PathTests.cpp
@@ -0,0 +1,36 @@
+//===-- PathTests.cpp -------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "support/Path.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace clang {
+namespace clangd {
+namespace {
+TEST(PathTests, IsAncestor) {
+  EXPECT_TRUE(pathIsAncestor("/foo", "/foo"));
+  EXPECT_TRUE(pathIsAncestor("/foo/", "/foo"));
+
+  EXPECT_FALSE(pathIsAncestor("/foo", "/fooz"));
+  EXPECT_FALSE(pathIsAncestor("/foo/", "/fooz"));
+
+  EXPECT_TRUE(pathIsAncestor("/foo", "/foo/bar"));
+  EXPECT_TRUE(pathIsAncestor("/foo/", "/foo/bar"));
+
+#ifdef CLANGD_PATH_CASE_INSENSITIVE
+  EXPECT_TRUE(pathIsAncestor("/fOo", "/foo/bar"));
+  EXPECT_TRUE(pathIsAncestor("/foo", "/fOo/bar"));
+#else
+  EXPECT_FALSE(pathIsAncestor("/fOo", "/foo/bar"));
+  EXPECT_FALSE(pathIsAncestor("/foo", "/fOo/bar"));
+#endif
+}
+} // namespace
+} // namespace clangd
+} // namespace clang