diff --git a/src/sidebar/components/MarkdownEditor.tsx b/src/sidebar/components/MarkdownEditor.tsx
index efb7f15dfbf..c00a91a9b2a 100644
--- a/src/sidebar/components/MarkdownEditor.tsx
+++ b/src/sidebar/components/MarkdownEditor.tsx
@@ -585,8 +585,8 @@ export default function MarkdownEditor({
   const input = useRef<HTMLTextAreaElement>(null);
 
   const textWithoutMentionTags = useMemo(
-    () => unwrapMentions(text, mentionMode),
-    [mentionMode, text],
+    () => unwrapMentions({ text, mentionMode, mentions }),
+    [mentionMode, mentions, text],
   );
 
   useEffect(() => {
diff --git a/src/sidebar/helpers/mentions.ts b/src/sidebar/helpers/mentions.ts
index 4b11496e85c..f7f6c712eef 100644
--- a/src/sidebar/helpers/mentions.ts
+++ b/src/sidebar/helpers/mentions.ts
@@ -96,27 +96,56 @@ export function wrapDisplayNameMentions(
 // some assumptions about the tag content in order to be able to get away
 // with it. Specifically we assume that these tags will not contain `<` or
 // `>` in attribute values or the HTML content of the tag.
-const MENTION_TAG_RE = /<a[^>]\bdata-hyp-mention\b[^>]*>([^<]+)<\/a>/g;
+const MENTION_TAG_RE = /<a[^>]\bdata-hyp-mention\b[^>]*>@([^<]+)<\/a>/g;
+
+export type UnwrapMentionsOptions = {
+  text: string;
+  mentionMode: MentionMode;
+  mentions?: Mention[];
+};
 
 /**
  * Replace all mentions wrapped in the special `<a data-hyp-mention />` tag with
  * their plain-text representation.
+ *
  * The plain-text representation depends on the mention mode:
  * - `username`: @username
  * - `display-name`: @[Display Name]
+ *
+ * If a list of mentions is provided, the tag's userid will be matched against
+ * it, so that the plain-text version uses the most recent username or display
+ * name, in case they have changed since the mention was created.
+ * If a list is not provided or the mention is not found, the tag's content will
+ * be used as username or display name.
  */
-export function unwrapMentions(text: string, mentionMode: MentionMode) {
+export function unwrapMentions({
+  text,
+  mentionMode,
+  mentions = [],
+}: UnwrapMentionsOptions) {
   // Use a regex rather than HTML parser to replace the mentions in order
   // to avoid modifying any of the content outside of the replaced tags. This
   // includes avoiding modifications such as encoding characters that will
   // happen when parsing and re-serializing HTML via eg. `innerHTML`.
-  return text.replace(MENTION_TAG_RE, (match, mention) => {
-    if (mentionMode === 'username') {
-      return mention;
-    }
+  return text.replace(MENTION_TAG_RE, (match, tagContent) => {
+    // Even though we need to capture the tag content via a regex for the
+    // reasons explained above, we can use regular DOM APIs to get the userid
+    const tempElement = document.createElement('div');
+    tempElement.innerHTML = match;
+    const userid = tempElement
+      .querySelector('a[data-hyp-mention]')
+      ?.getAttribute('data-userid');
+
+    const mention = mentions.find(
+      ({ original_userid }) => original_userid === userid,
+    );
 
-    const [atChar, ...rest] = mention;
-    return `${atChar}[${rest.join('')}]`;
+    return mentionMode === 'username'
+      ? `@${mention?.username ?? tagContent}`
+      : // Using || rather than ?? for the display name, to avoid setting an
+        // empty string if the user used to have a display name and has been
+        // removed since the mention was created
+        `@[${mention?.display_name || tagContent}]`;
   });
 }
 
diff --git a/src/sidebar/helpers/test/mentions-test.js b/src/sidebar/helpers/test/mentions-test.js
index 591e465f5ef..3abb14db8e5 100644
--- a/src/sidebar/helpers/test/mentions-test.js
+++ b/src/sidebar/helpers/test/mentions-test.js
@@ -134,7 +134,10 @@ Hello ${mentionTag('jane.doe', 'example.com')}.`,
 
   describe('unwrapMentions - `username` mode', () => {
     it('removes wrapping mention tags', () => {
-      assert.equal(unwrapMentions(textWithTags, 'username'), text);
+      assert.equal(
+        unwrapMentions({ text: textWithTags, mentionMode: 'username' }),
+        text,
+      );
     });
   });
 });
@@ -211,7 +214,88 @@ Hello ${mentionTag('jane.doe', 'example.com')}.`,
 
   describe('unwrapMentions - `display-name` mode', () => {
     it('removes wrapping mention tags', () => {
-      assert.equal(unwrapMentions(textWithTags, 'display-name'), text);
+      assert.equal(
+        unwrapMentions({ text: textWithTags, mentionMode: 'display-name' }),
+        text,
+      );
+    });
+  });
+});
+
+describe('unwrapMentions', () => {
+  [
+    // Mention not found. Tag content kept
+    {
+      mentionMode: 'username',
+      mentions: [],
+      text: `Hello ${mentionTag('jane_doe')}`,
+      expectedResult: 'Hello @jane_doe',
+    },
+    {
+      mentionMode: 'display-name',
+      mentions: [],
+      text: `Hello ${displayNameMentionTag('Jane Doe', 'jane_doe')}`,
+      expectedResult: 'Hello @[Jane Doe]',
+    },
+
+    // Mention found with a new username
+    {
+      mentionMode: 'username',
+      mentions: [
+        {
+          original_userid: 'acct:jane_doe@hypothes.is',
+          username: 'jane_edited',
+        },
+      ],
+      text: `Hello ${mentionTag('jane_doe')}`,
+      expectedResult: 'Hello @jane_edited',
+    },
+
+    // Mention found with a new display name
+    {
+      mentionMode: 'display-name',
+      mentions: [
+        {
+          original_userid: 'acct:jane_doe@hypothes.is',
+          display_name: 'My new name',
+        },
+      ],
+      text: `Hello ${displayNameMentionTag('Jane Doe', 'jane_doe')}`,
+      expectedResult: 'Hello @[My new name]',
+    },
+
+    // Mention found with a now empty display name
+    {
+      mentionMode: 'display-name',
+      mentions: [
+        {
+          original_userid: 'acct:jane_doe@hypothes.is',
+          display_name: '',
+        },
+      ],
+      text: `Hello ${displayNameMentionTag('Jane Doe', 'jane_doe')}`,
+      expectedResult: 'Hello @[Jane Doe]',
+    },
+
+    // data-userid not present
+    {
+      mentionMode: 'username',
+      mentions: [],
+      text: 'Hello <a data-hyp-mention="">@user_id_missing</a>',
+      expectedResult: 'Hello @user_id_missing',
+    },
+    {
+      mentionMode: 'display-name',
+      mentions: [],
+      text: 'Hello <a data-hyp-mention="">@User ID Missing</a>',
+      expectedResult: 'Hello @[User ID Missing]',
+    },
+  ].forEach(({ text, mentionMode, mentions, expectedResult }) => {
+    it('replaces mention tag with current username or display name', () => {
+      assert.equal(
+        unwrapMentions({ text, mentionMode, mentions }),
+        expectedResult,
+      );
     });
   });
 });