diff --git a/h/presenters/mention_json.py b/h/presenters/mention_json.py
index ebd61188f41..b0098f37f53 100644
--- a/h/presenters/mention_json.py
+++ b/h/presenters/mention_json.py
@@ -1,6 +1,7 @@
 from typing import Any
 
 from h.models import Mention
+from h.util.user import format_userid
 
 
 class MentionJSONPresenter:
@@ -12,7 +13,10 @@ def __init__(self, mention: Mention):
     def asdict(self) -> dict[str, Any]:
         return {
             "userid": self._mention.user.userid,
-            "username": self._mention.username,
+            "original_userid": format_userid(
+                self._mention.username, self._mention.user.authority
+            ),
+            "username": self._mention.user.username,
             "display_name": self._mention.user.display_name,
             "link": self._mention.user.uri,
         }
diff --git a/tests/unit/h/presenters/mention_json_test.py b/tests/unit/h/presenters/mention_json_test.py
index 23cf9db902f..b5251c9f914 100644
--- a/tests/unit/h/presenters/mention_json_test.py
+++ b/tests/unit/h/presenters/mention_json_test.py
@@ -2,6 +2,7 @@
 
 from h.models import Mention
 from h.presenters.mention_json import MentionJSONPresenter
+from h.util.user import format_userid
 
 
 class TestMentionJSONPresenter:
@@ -12,11 +13,20 @@ def test_as_dict(self, user, annotation):
 
         assert data == {
             "userid": user.userid,
+            "original_userid": user.userid,
             "username": user.username,
             "display_name": user.display_name,
             "link": user.uri,
         }
 
+    def test_as_dict_with_different_username(self, user, annotation):
+        new_username = "new_username"
+        mention = Mention(annotation=annotation, user=user, username=new_username)
+
+        data = MentionJSONPresenter(mention).asdict()
+
+        assert data["original_userid"] == format_userid(new_username, user.authority)
+
     @pytest.fixture
     def user(self, factories):
         return factories.User.build()
diff --git a/tests/unit/h/services/mention_test.py b/tests/unit/h/services/mention_test.py
index 5ca404353e5..480dc9935c9 100644
--- a/tests/unit/h/services/mention_test.py
+++ b/tests/unit/h/services/mention_test.py
@@ -104,6 +104,7 @@ def test_it(self, pyramid_request, user_service, MentionService):
             session=pyramid_request.db,
             user_service=user_service,
         )
+
         assert service == MentionService.return_value
 
     @pytest.fixture