Eventually The Brady Bunch begat The Bradys an hour-long drama that lasted just six epsiodes.
Hill Street Blues (fondly remembered drama) spun off Beverly Hills Buntz (completely forgotten comedy)
Vince Vaughn started his career playing villains. He’s now moved pretty much entirely to comedy.
He was much funnier in “Gun Shy”.
McHale’s Navy was a “spinoff” of a dramatic, gritty war drama called “Seven Against the Sea”
Alex Karras would also become known to younger audiences as the dad, George Papadapolous, on the sitcom Webster.
Another ex-boxer who was successful at comedy was Rocky Graziano. He was frequently on talk shows like Merv Griffin and Mike Douglas in the '60s & '70s, always telling rambling stories/anecdotes. A very young Me always found him funny and my mother laughed too, so I guess he had cross-generational appeal. 
Google tells me that shortly after his retirement from boxing he co-hosted a 15-minute show called The Henny and Rocky Show that followed broadcast boxing matches. Henny was Henny Youngman. I would love to have seen that one.
Speaking of boxers and (unintentional) comedy, the inspiration for the Simpsons’ runner where Bart calls Moe’s Tavern and asks to speak to fake names (Al Caholic, Seymour Butts, etc.) is based on a couple of pranksters from the 70s who did the same thing. They would call up Louis “Red” Deutsch, the proprietor of a bar in New Jersey and (like Moe) an ex-boxer, asking to speak with gag names that Red would call out to the bar. When he caught on, Red would profusely curse out the pranksters.
The British actor Edward Woodward was famous for his roles in crime and spy thrillers such as ‘The Equalizer’ and ‘Callan’. Always the dark avenger.
In later life he showed his range with a role in a sit-com about garbage collectors. ‘Common as muck’.
Come to think of it, how about Shatner?
Thank you for Googling this. It makes sense.
Right, on Boston Legal, Shatner and James Spader (not, I think known for many comedic roles before that) handled the show’s mix of drama and comedy pretty well.
William Shatner also had a musical string to his bow.
His rendition of the Beatles hit ‘Lucy in the Sky with Diamonds’ was highly original. The listener was challenged to discern the pathos from the bathos through a kaleidoscope of beguiling psychedelia. What was he smoking?
Shatner had many talents and some of them were not sufficiently hidden from the public.
Shatner’s turn hosting Saturday Night Live in 1986 was hilarious, including the infamous “get a life” sketch set at a Trek convention.
Ray Liotta has shown a flair for comedy on SNL and movies when he gets the chance.
Terry Crews started as a professional football player in the NFL, and has since gone on to a successful comedic acting and hosting career.
I watched a little bit of the first episode of afterMASH on YouTube just out of curiosity. I got about 6 minutes in when I saw this cringeworthy exchange- the setup: Potter has a happy reunion with Mrs. Potter, but finds retired life a little boring, so he decides to do some part time practicing as a G.P. A female patient comes in…
Dr. Potter: take your clothes off.
Patient: can I have a nurse present?
Potter (indignantly): I’ve already seen you naked! I cut your umbilical cord!
Patient: but I’m a grown woman now.
Potter: so what? The parts are all the same, some of them just swelled up a little!
Patient: still, I’d feel more comfortable with a nurse present.
Potter (shouting now): if you make me call a nurse in, I’m going to make her take her clothes off too!
I mean, I know it was a different time, and part of the joke was his attitude was old fashioned even then, but wow.
That was a great episode. Cold open was “The Mute Marine.” They also had the lost ending to “It’s A Wonderful Life” and the Enterprise as a rotating restaurant. Even the TJ Hooker parody’s one joke hit.
Plus, Lone Justice was the musical guest!
I’d say Sidney Poitier. Primarily a dramatic actor, in the '70’s he did a number of comedies, often paired with Bill Cosby.
StG
I think De Niro’s best comedy by a long way is Midnight Run with Charles Grodin. One of my favorite movies of the 80s and one of the best buddy/road movies ever.
I submit George C. Scott. It was more mid-career, rather than late, but his over-the-top character, General ‘Buck’ Turgidson, in the black comedy Dr. Strangelove (one of my favorite movies) is one of the funniest characters in film history, IMHO. Scott was pushed hard by Kubrick to over-act the role. The result is a brilliant comedic performance by one of Hollywood’s finest dramatic actors. I get the giggles just thinking about him!
Dramatic actor Sterling Hayden was pretty damned funny as Brig. Gen. Jack D. Ripper in that movie, too.
And, of course, Peter Sellers was always a stellar comedian. He played 3 characters in Strangelove (actually planned to do 4, but backed out of the 4th).