diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -33,6 +33,7 @@
 #include "llvm/Support/xxhash.h"
 
 #include <algorithm>
+#include <future>
 
 using namespace llvm;
 using namespace llvm::MachO;
@@ -1147,9 +1148,13 @@
   sortSegmentsAndSections();
   createLoadCommands<LP>();
   finalizeAddresses();
+  std::future<void> writeMapAsync(std::async(writeMapFile));
   finalizeLinkEditSegment();
-  writeMapFile();
   writeOutputFile();
+
+  // Ensure writeMapFile completes before the main thread returns
+  // to prevent undefined behavior.
+  writeMapAsync.get();
 }
 
 template <class LP> void macho::writeResult() { Writer().run<LP>(); }