diff --git a/chrome/updater/policy/service.cc b/chrome/updater/policy/service.cc
index a7fb3843f1b02..53c0f08c06e89 100644
--- a/chrome/updater/policy/service.cc
+++ b/chrome/updater/policy/service.cc
@@ -131,12 +131,14 @@ bool PolicyService::PolicyManagers::CloudPolicyOverridesPlatformPolicy(
 
 void PolicyService::PolicyManagers::ResetDeviceManagementManager(
     scoped_refptr<PolicyManagerInterface> dm_manager) {
+  base::AutoLock lock(managers_lock_);
   dm_policy_manager_ = dm_manager;
   InitializeManagersVector();
 }
 
 void PolicyService::PolicyManagers::SetManagersForTesting(
     std::vector<scoped_refptr<PolicyManagerInterface>> managers) {
+  base::AutoLock lock(managers_lock_);
   managers_ = std::move(managers);
   SortManagersVector();
 }
diff --git a/chrome/updater/policy/service.h b/chrome/updater/policy/service.h
index ede8beaa90ecc..c8e3fe48cad28 100644
--- a/chrome/updater/policy/service.h
+++ b/chrome/updater/policy/service.h
@@ -16,6 +16,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_refptr.h"
 #include "base/sequence_checker.h"
+#include "base/synchronization/lock.h"
 #include "base/strings/string_util.h"
 #include "base/time/time.h"
 #include "base/values.h"
@@ -159,6 +160,11 @@ class PolicyService : public base::RefCountedThreadSafe<PolicyService> {
         scoped_refptr<PolicyManagerInterface> dm_manager);
 
     std::vector<scoped_refptr<PolicyManagerInterface>> managers() const {
+      // Called on arbitrary COM RPC threads (see crbug.com/1517079) while
+      // `ResetDeviceManagementManager()` mutates `managers_` on the main
+      // sequence. Take the lock so the by-value copy is atomic w.r.t. the
+      // clear()/rebuild in `InitializeManagersVector()`.
+      base::AutoLock lock(managers_lock_);
       return managers_;
     }
 
@@ -172,6 +178,11 @@ class PolicyService : public base::RefCountedThreadSafe<PolicyService> {
     static bool CloudPolicyOverridesPlatformPolicy(
         const std::vector<scoped_refptr<PolicyManagerInterface>>& providers);
 
+    // Guards `managers_` against concurrent reads (COM RPC threads via
+    // `managers()`) and writes (`ResetDeviceManagementManager()` /
+    // `InitializeManagersVector()` / `SetManagersForTesting()` on the main
+    // sequence). See crbug.com/1517079.
+    mutable base::Lock managers_lock_;
     std::vector<scoped_refptr<PolicyManagerInterface>> managers_;
     scoped_refptr<PolicyManagerInterface> dm_policy_manager_;
     scoped_refptr<PolicyManagerInterface> external_constants_policy_manager_;