The wonderful film about a genius who worked as a cleaner at a university who was able to solve incredibly difficult mathematical problems was Good Will Hunting, starring Matt Damon.
A Beautiful Mind was a movie about gifted mathematician John Nash starring Russell Crowe. John Nash got his Phd in non-cooperative games (including the Nash Equlibrium) at age 22. I'm pretty sure neither Nash nor Crowe worked as a janitor.
