diff --git a/ident.c b/ident.c
index 967895d885033da5393a9c0c0c52d51531a64867..281e830573b74d3194f62436b15f1d1d8a33056a 100644
--- a/ident.c
+++ b/ident.c
@@ -412,6 +412,10 @@ void apply_mailmap_to_header(struct strbuf *buf, const char **header,
 				found_header = 1;
 				buf_offset += endp - line;
 				buf_offset += rewrite_ident_line(person, endp - person, buf, mailmap);
+				/* Recompute endp after potential buffer reallocation */
+				endp = buf->buf + buf_offset;
+				if (*endp == '\n')
+					buf_offset++;
 				break;
 			}
 
diff --git a/t/t4203-mailmap.sh b/t/t4203-mailmap.sh
index 4a6242ff99b59ea1a46eb14ca812c94e5e620162..98dd0ae12f7da50a81f472f005b4daf5bf79e662 100755
--- a/t/t4203-mailmap.sh
+++ b/t/t4203-mailmap.sh
@@ -1133,4 +1133,37 @@ test_expect_success 'git cat-file --batch-command returns correct size with --us
 	test_cmp expect actual
 '
 
+test_expect_success 'git cat-file --mailmap works with different author and committer' '
+	test_when_finished "rm .mailmap" &&
+	cat >.mailmap <<-\EOF &&
+	Mailmapped User <mailmapped-user@gitlab.com> C O Mitter <committer@example.com>
+	EOF
+	git commit --allow-empty -m "different author/committer" \
+		--author="Different Author <different@example.com>" &&
+	cat >expect <<-\EOF &&
+	author Different Author <different@example.com>
+	committer Mailmapped User <mailmapped-user@gitlab.com>
+	EOF
+	git cat-file --mailmap commit HEAD >log &&
+	sed -n "/^author /s/\([^>]*>\).*/\1/p; /^committer /s/\([^>]*>\).*/\1/p" log >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'git cat-file --mailmap maps both author and committer when both need mapping' '
+	test_when_finished "rm .mailmap" &&
+	cat >.mailmap <<-\EOF &&
+	Mapped Author <mapped-author@example.com> <different@example.com>
+	Mapped Committer <mapped-committer@example.com> C O Mitter <committer@example.com>
+	EOF
+	git commit --allow-empty -m "both author and committer mapped" \
+		--author="Different Author <different@example.com>" &&
+	cat >expect <<-\EOF &&
+	author Mapped Author <mapped-author@example.com>
+	committer Mapped Committer <mapped-committer@example.com>
+	EOF
+	git cat-file --mailmap commit HEAD >log &&
+	sed -n "/^author /s/\([^>]*>\).*/\1/p; /^committer /s/\([^>]*>\).*/\1/p" log >actual &&
+	test_cmp expect actual
+'
+
 test_done