diff --git a/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp b/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
--- a/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
+++ b/clang-tools-extra/clang-tidy/llvm/IncludeOrderCheck.cpp
@@ -10,6 +10,7 @@
 #include "clang/Frontend/CompilerInstance.h"
 #include "clang/Lex/PPCallbacks.h"
 #include "clang/Lex/Preprocessor.h"
+#include "llvm/ADT/STLExtras.h"
 
 #include <map>
 
@@ -61,16 +62,24 @@
   if (IsMainModule)
     return 0;
 
-  // LLVM and clang headers are in the penultimate position.
-  if (Filename.startswith("llvm/") || Filename.startswith("llvm-c/") ||
-      Filename.startswith("clang/") || Filename.startswith("clang-c/"))
+  auto FilenameStartsWith = [&](StringRef Search) {
+    return Filename.startswith(Search);
+  };
+
+  constexpr StringRef LLVMClangHeaders[] = {"llvm/", "llvm-c/", "clang/",
+                                            "clang-c/"};
+  constexpr StringRef OtherHeaders[] = {"gtest/", "gmock/", "isl/", "json/"};
+
+  if (llvm::any_of(LLVMClangHeaders, FilenameStartsWith))
     return 2;
 
-  // System headers are sorted to the end.
-  if (IsAngled || Filename.startswith("gtest/") ||
-      Filename.startswith("gmock/"))
+  if (llvm::any_of(OtherHeaders, FilenameStartsWith))
     return 3;
 
+  // System headers are sorted to the end.
+  if (IsAngled)
+    return 4;
+
   // Other headers are inserted between the main module header and LLVM headers.
   return 1;
 }
diff --git a/clang-tools-extra/test/clang-tidy/checkers/llvm-include-order.cpp b/clang-tools-extra/test/clang-tidy/checkers/llvm-include-order.cpp
--- a/clang-tools-extra/test/clang-tidy/checkers/llvm-include-order.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/llvm-include-order.cpp
@@ -6,6 +6,7 @@
 #include "gmock/foo.h"
 #include "i.h"
 #include <s.h>
+#include <a.h>
 #include "llvm/a.h"
 #include "clang/b.h"
 #include "clang-c/c.h" // hi
@@ -19,6 +20,7 @@
 // CHECK-FIXES-NEXT: #include "llvm/a.h"
 // CHECK-FIXES-NEXT: #include "gmock/foo.h"
 // CHECK-FIXES-NEXT: #include "gtest/foo.h"
+// CHECK-FIXES-NEXT: #include <a.h>
 // CHECK-FIXES-NEXT: #include <s.h>
 
 #include "b.h"