Board Thread:General Discussion/@comment-43860040-20190919052920/@comment-366087-20190919095732

Before marriage, Penny's last name was *never* provided. This is confirmed by the show's writers, also covered in a Late Show with Steven Colbert interview. (sorry, too tired, lazy, and uninterested to find any links).

Penny does not have a maiden/nee/family name/last name/surname before Hofstadter. If you've heard any other, it is likely a fanon-invention, thus apocryphal and non-canon.