Mayim Bialik has a PhD in neuroscience, and plays a neuroscientist (Amy Farrah Fowler) on The Big Bang Theory.
Ben Jones, who played Cooter on The Dukes of Hazzard, was elected to the House of Representatives from Georgia after being on the show. When the Dukes reunion TV-movie came out, it was said that Cooter got elected to Congress from the state of Georgia.
Dick Butkus played football-player characters in several movies, including THE LONGEST YARD, NECESSARY ROUGHNESS, and ANY GIVEN SUNDAY, and also one on THE SIX MILLION DOLLAR MAN.
Figure skater Lynn-Holly Johnson played a figure skater in FOR YOUR EYES ONLY.
Boxer Tommy Morrison, who just died the other day, played a boxer in ROCKY V.
Tons of people showed up in Paper Lion(Vince Lombardi, Sugar Ray Robinson and the unbilled voice of Ernie Harwell) as did many of the real life Detroit Lions of the time including the late great Alex Karras, who played himself.
I believe Jamie Farr (Corporal Klinger) was the only cast member of the M*A*S*H TV show to have actually served in the U.S. Army in South Korea (albeit a few years after the war ended). He wore his actual dog tags on the show.