diff --git a/src/win/conpty.cc b/src/win/conpty.cc
index 633102d7f..66741d9d7 100644
--- a/src/win/conpty.cc
+++ b/src/win/conpty.cc
@@ -14,6 +14,8 @@
 #include <node_api.h>
 #include <assert.h>
 #include <Shlwapi.h> // PathCombine, PathIsRelative
+#include <atomic>
+#include <mutex>
 #include <sstream>
 #include <iostream>
 #include <string>
@@ -50,9 +52,12 @@ struct pty_baton {
 };
 
 static std::vector<std::unique_ptr<pty_baton>> ptyHandles;
-static volatile LONG ptyCounter;
+static std::mutex g_ptyHandlesMutex;
+static std::atomic<int> ptyCounter{0};
 
-static pty_baton* get_pty_baton(int id) {
+// The leading scoped-lock parameter encodes the precondition that the caller
+// holds g_ptyHandlesMutex.
+static pty_baton* get_pty_baton(const std::lock_guard<std::mutex>&, int id) {
   auto it = std::find_if(ptyHandles.begin(), ptyHandles.end(), [id](const auto& ptyHandle) {
     return ptyHandle->id == id;
   });
@@ -62,17 +67,6 @@ static pty_baton* get_pty_baton(int id) {
   return nullptr;
 }
 
-static bool remove_pty_baton(int id) {
-  auto it = std::remove_if(ptyHandles.begin(), ptyHandles.end(), [id](const auto& ptyHandle) {
-    return ptyHandle->id == id;
-  });
-  if (it != ptyHandles.end()) {
-    ptyHandles.erase(it);
-    return true;
-  }
-  return false;
-}
-
 struct ExitEvent {
   int exit_code = 0;
 };
@@ -99,11 +93,15 @@ void SetupExitCallback(Napi::Env env, Napi::Function cb, pty_baton* baton) {
     ExitEvent *exit_event = new ExitEvent;
     // Wait for process to complete.
     WaitForSingleObject(baton->hShell, INFINITE);
-    // Get process exit code.
-    GetExitCodeProcess(baton->hShell, (LPDWORD)(&exit_event->exit_code));
-    // Clean up handles
-    CloseHandle(baton->hShell);
-    assert(remove_pty_baton(baton->id));
+    {
+      std::lock_guard<std::mutex> lock(g_ptyHandlesMutex);
+      GetExitCodeProcess(baton->hShell, (LPDWORD)(&exit_event->exit_code));
+      CloseHandle(baton->hShell);
+      const int id = baton->id;
+      std::erase_if(ptyHandles, [id](const auto& ptyHandle) {
+        return ptyHandle->id == id;
+      });
+    }
 
     auto status = tsfn.BlockingCall(exit_event, callback); // In main thread
     switch (status) {
@@ -298,10 +296,13 @@ static Napi::Value PtyStartProcess(const Napi::CallbackInfo& info) {
 
   if (SUCCEEDED(hr)) {
     // We were able to instantiate a conpty
-    const int ptyId = InterlockedIncrement(&ptyCounter);
+    const int ptyId = ++ptyCounter;
     marshal.Set("pty", Napi::Number::New(env, ptyId));
-    ptyHandles.emplace_back(
-        std::make_unique<pty_baton>(ptyId, hIn, hOut, hpc));
+    {
+      std::lock_guard<std::mutex> lock(g_ptyHandlesMutex);
+      ptyHandles.emplace_back(
+          std::make_unique<pty_baton>(ptyId, hIn, hOut, hpc));
+    }
   } else {
     throw Napi::Error::New(env, "Cannot launch conpty");
   }
@@ -349,10 +350,13 @@ static Napi::Value PtyConnect(const Napi::CallbackInfo& info) {
   const bool useConptyDll = info[4].As<Napi::Boolean>().Value();
   Napi::Function exitCallback = info[5].As<Napi::Function>();
 
-  // Fetch pty handle from ID and start process
-  pty_baton* handle = get_pty_baton(id);
-  if (!handle) {
-    throw Napi::Error::New(env, "Invalid pty handle");
+  pty_baton* handle;
+  {
+    std::lock_guard<std::mutex> lock(g_ptyHandlesMutex);
+    handle = get_pty_baton(lock, id);
+    if (!handle) {
+      throw Napi::Error::New(env, "Invalid pty handle");
+    }
   }
 
   // Prepare command line
@@ -471,7 +475,8 @@ static Napi::Value PtyResize(const Napi::CallbackInfo& info) {
   SHORT rows = static_cast<SHORT>(info[2].As<Napi::Number>().Uint32Value());
   const bool useConptyDll = info[3].As<Napi::Boolean>().Value();
 
-  const pty_baton* handle = get_pty_baton(id);
+  std::lock_guard<std::mutex> lock(g_ptyHandlesMutex);
+  const pty_baton* handle = get_pty_baton(lock, id);
 
   if (handle != nullptr) {
     HANDLE hLibrary = LoadConptyDll(info, useConptyDll);
@@ -512,7 +517,8 @@ static Napi::Value PtyClear(const Napi::CallbackInfo& info) {
     return env.Undefined();
   }
 
-  const pty_baton* handle = get_pty_baton(id);
+  std::lock_guard<std::mutex> lock(g_ptyHandlesMutex);
+  const pty_baton* handle = get_pty_baton(lock, id);
 
   if (handle != nullptr) {
     HANDLE hLibrary = LoadConptyDll(info, useConptyDll);
@@ -543,7 +549,8 @@ static Napi::Value PtyKill(const Napi::CallbackInfo& info) {
   int id = info[0].As<Napi::Number>().Int32Value();
   const bool useConptyDll = info[1].As<Napi::Boolean>().Value();
 
-  const pty_baton* handle = get_pty_baton(id);
+  std::lock_guard<std::mutex> lock(g_ptyHandlesMutex);
+  const pty_baton* handle = get_pty_baton(lock, id);
 
   if (handle != nullptr) {
     HANDLE hLibrary = LoadConptyDll(info, useConptyDll);
diff --git a/src/windowsTerminal.test.ts b/src/windowsTerminal.test.ts
index 28c102bcb..303086ae4 100644
--- a/src/windowsTerminal.test.ts
+++ b/src/windowsTerminal.test.ts
@@ -268,6 +268,58 @@ if (process.platform === 'win32') {
           });
         });
       });
+
+      describe('Regression for #921', () => {
+        it('should not crash with concurrent kills while resizing/clearing', function (done) {
+          this.timeout(60000);
+          const N = 30;
+          const terms: WindowsTerminal[] = [];
+          let ready = 0;
+          let exited = 0;
+          let spamInterval: NodeJS.Timeout | undefined;
+          const cleanup = (err?: Error): void => {
+            if (spamInterval) {
+              clearInterval(spamInterval);
+              spamInterval = undefined;
+            }
+            done(err);
+          };
+          const startRace = (): void => {
+            spamInterval = setInterval(() => {
+              for (const t of terms) {
+                try {
+                  t.resize(80 + Math.floor(Math.random() * 40), 24 + Math.floor(Math.random() * 20));
+                } catch (e) { /* already exited */ }
+                try {
+                  t.clear();
+                } catch (e) { /* already exited */ }
+              }
+            }, 1);
+            for (const t of terms) {
+              try { t.kill(); } catch (e) { /* */ }
+            }
+          };
+          for (let i = 0; i < N; i++) {
+            const t = new WindowsTerminal('cmd.exe', [], { useConptyDll });
+            terms.push(t);
+            let readied = false;
+            t.on('data', () => {
+              if (readied) return;
+              readied = true;
+              ready++;
+              if (ready === N) {
+                startRace();
+              }
+            });
+            t.on('exit', () => {
+              exited++;
+              if (exited === N) {
+                cleanup();
+              }
+            });
+          }
+        });
+      });
     });
   });
 }