My father owns rental property which he is trying to sell. He has owned it for years (decades). He had a buyer and during the transaction learned that his real estate attorney (long since deceased) never filed the deed/title. He no longer has the paperwork from that transaction. Does he need to file a quiet title action to obtain official title of his own property?

So who's name is the deed in? How long has your father owned the property? Did he buy it outright? Does he have any documentation to prove ownership?

My father ... no longer has the paperwork from that transaction. Does he need to file a quiet title action to obtain official title of his own property?

As I understand what you are saying, your father does not have any "color of title" (i.e.: the deed). If your jurisdiction's adverse possession statutes allow one to acquire title by adverse possession without color of title, yes, that is probably what he will need to do. He needs to consult with an attorney in his jurisdiction.

OK - thanks!