Sandun found Penny maiden name.
However, during a good part of the story, before she marries Leonard, her legal name was Johnson.
Indeed she did marry Zack Johnson in Las Vegas, and even though both thought the wedding was just for fun, the marriage was in fact perfectly valid. This was told in The Thanksgiving Decoupling, season 7 episode 9. The wedding itself is not in any episode, but in the Trivia section of the page I am linking they say this happened three years before that episode, so she was indeed Mrs. Penny Johnson for three years.
Penny needs a proper divorce from Zack before she could marry Leonard, and after some hesitations Zack agrees to sign the divorce papers at the end of that same episode, so Penny can legally marry Leonard a bit later.