diff --git a/lib/shared/mention-utils.js b/lib/shared/mention-utils.js
--- a/lib/shared/mention-utils.js
+++ b/lib/shared/mention-utils.js
@@ -26,6 +26,16 @@
   return new RegExp(`\\B@${username}\\b`, 'i').test(text);
 }
 
+const mentionsExtractionRegex = new RegExp(
+  `\\B(@(${oldValidUsernameRegexString}))\\b`,
+  'g',
+);
+
+function extractMentionsFromText(text: string): string[] {
+  const iterator = text.matchAll(mentionsExtractionRegex);
+  return [...iterator].map(matches => matches[2]);
+}
+
 function getTypeaheadRegexMatches(
   text: string,
   selection: Selection,
@@ -82,6 +92,7 @@
 export {
   markdownMentionRegex,
   isMentioned,
+  extractMentionsFromText,
   getTypeaheadUserSuggestions,
   getNewTextAndSelection,
   getTypeaheadRegexMatches,
diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -7,6 +7,7 @@
 import tinycolor from 'tinycolor2';
 
 import { type ParserRules } from './markdown.js';
+import { extractMentionsFromText } from './mention-utils.js';
 import { getMessageTitle } from './message-utils.js';
 import { relationshipBlockedInEitherDirection } from './relationship-utils.js';
 import threadWatcher from './thread-watcher.js';
@@ -42,9 +43,10 @@
   usersWithPersonalThreadSelector,
 } from '../selectors/user-selectors.js';
 import type { CalendarQuery } from '../types/entry-types.js';
-import type {
-  RobotextMessageInfo,
-  ComposableMessageInfo,
+import {
+  type RobotextMessageInfo,
+  type ComposableMessageInfo,
+  messageTypes,
 } from '../types/message-types.js';
 import { userRelationshipStatus } from '../types/relationship-types.js';
 import {
@@ -462,8 +464,10 @@
   const threadName = trimText(messageTitle, 30);
 
   const initialMembers = new Map();
-  const { id: viewerID, username } = loggedInUserInfo;
-  initialMembers.set(viewerID, { id: viewerID, username });
+
+  const { id: viewerID, username: viewerUsername } = loggedInUserInfo;
+  initialMembers.set(viewerID, { id: viewerID, username: viewerUsername });
+
   if (userIsMember(parentThreadInfo, sourceMessageInfo.creator.id)) {
     const {
       id: sourceAuthorID,
@@ -479,6 +483,7 @@
     };
     initialMembers.set(sourceAuthorID, initialMemberUserInfo);
   }
+
   const singleOtherUser = getSingleOtherUser(parentThreadInfo, viewerID);
   if (parentThreadType === threadTypes.PERSONAL && singleOtherUser) {
     const singleOtherUsername = parentThreadInfo.members.find(
@@ -495,6 +500,24 @@
     initialMembers.set(singleOtherUser, singleOtherUserInfo);
   }
 
+  if (sourceMessageInfo.type === messageTypes.TEXT) {
+    const memberMap = new Map();
+    for (const member of parentThreadInfo.members) {
+      const { id, role, username } = member;
+      if (!role || !username) {
+        continue;
+      }
+      memberMap.set(username.toLowerCase(), { id, username });
+    }
+    const mentions = extractMentionsFromText(sourceMessageInfo.text);
+    for (const mention of mentions) {
+      const userInfo = memberMap.get(mention.toLowerCase());
+      if (userInfo) {
+        initialMembers.set(userInfo.id, userInfo);
+      }
+    }
+  }
+
   return createPendingThread({
     viewerID,
     threadType: threadTypes.SIDEBAR,