diff --git a/sharing/certificates/nearby_share_certificate_manager_impl.cc b/sharing/certificates/nearby_share_certificate_manager_impl.cc
index 42a23976..00154aad 100644
--- a/sharing/certificates/nearby_share_certificate_manager_impl.cc
+++ b/sharing/certificates/nearby_share_certificate_manager_impl.cc
@@ -406,11 +406,10 @@ bool NearbyShareCertificateManagerImpl::DownloadPublicCertificatesInExecutor() {
   }
 
   bool download_succeeded = false;
-  // Currently certificates download is synchronous.  It completes after
-  // QuerySharedCredentialsFetchNextPage() returns.
+  absl::Notification notification;
   auto context = std::make_unique<CertificateDownloadContext>(
       nearby_identity_client_.get(), std::move(device_id),
-      [this, &download_succeeded](
+      [this, &download_succeeded, &notification](
           absl::StatusOr<std::vector<PublicCertificate>> certificates_status) {
         if (!certificates_status.ok()) {
           download_succeeded = false;
@@ -426,8 +425,13 @@ bool NearbyShareCertificateManagerImpl::DownloadPublicCertificatesInExecutor() {
           }
           download_succeeded = UpdatePublicCertificates(certificates);
         }
+        notification.Notify();
       });
   context->QuerySharedCredentialsFetchNextPage();
+  // Wait for all pages of certificates to be downloaded.
+  // MUST not terminate early, otherwise notification will go out of scope, and
+  // the callback will call Notify on a destroyed object.
+  notification.WaitForNotification();
   LOG(INFO) << "Public certificates downloadws, success: "
             << download_succeeded;
   return download_succeeded;
@@ -551,6 +555,8 @@ bool NearbyShareCertificateManagerImpl::UploadDeviceCertificatesInExecutor(
         }
         notification.Notify();
       });
+  // MUST not terminate early, otherwise notification will go out of scope, and
+  // the callback will call Notify on a destroyed object.
   notification.WaitForNotification();
   // check whether the manager is still running
   if (!is_running()) {
diff --git a/sharing/contacts/nearby_share_contact_manager_impl.cc b/sharing/contacts/nearby_share_contact_manager_impl.cc
index c179f300..8ad1dc7f 100644
--- a/sharing/contacts/nearby_share_contact_manager_impl.cc
+++ b/sharing/contacts/nearby_share_contact_manager_impl.cc
@@ -25,6 +25,7 @@
 
 #include "absl/memory/memory.h"
 #include "absl/status/statusor.h"
+#include "absl/synchronization/notification.h"
 #include "internal/platform/implementation/account_manager.h"
 #include "sharing/contacts/nearby_share_contact_manager.h"
 #include "sharing/internal/api/sharing_rpc_client.h"
@@ -135,11 +136,22 @@ void NearbyShareContactManagerImpl::GetContacts(ContactsCallback callback) {
       return;
     }
 
-    // Currently Contacts download is synchronous.  It completes after
-    // FetchNextPage() returns.
+    absl::Notification notification;
     auto context = std::make_unique<ContactDownloadContext>(
-        nearby_share_client_.get(), std::move(callback));
+        nearby_share_client_.get(),
+        [&notification, callback = std::move(callback)](
+            absl::StatusOr<std::vector<nearby::sharing::proto::ContactRecord>>
+                contacts,
+            uint32_t num_unreachable_contacts_filtered_out) mutable {
+          std::move(callback)(std::move(contacts),
+                              num_unreachable_contacts_filtered_out);
+          notification.Notify();
+        });
     context->FetchNextPage();
+    // Wait for all pages of contacts to be downloaded.
+    // MUST not terminate early, otherwise notification will go out of scope,
+    // and the callback will call Notify on a destroyed object.
+    notification.WaitForNotification();
   });
 }