Ashton Kurshner–That 70’s Show & 2 1/2 Men.
Debra Jo Rupp–That 70’s Show & Friends
Far too many to count. I’ll start with Ronnie Barker, David Jason, Arthur Lowe, Clive Dunn, Paul Eddington, Richard Briers, Rowan Atkinson, etc etc.
Technically, The Andy Griffith Show is a spin-off of The Danny Thomas Show, but I think we can allow Don Knotts and Ron Howard on the grounds that their successful second shows were not spun off of their first ones, nor were they reprising a character.
Heck, wasn’t Happy Days a spin-off of yet another show?
Ron Howard was also in the 1959 Dennis the Menace series.
Yes, Love American Style. And of course Happy Days also spun off Laverne and Shirley and Mork and Mindy and Joanie Loves Chachi and some show about an angel that didn’t last long and also the Happy Days cartoon.
Indeed, Patricia Heaton was on the wildly successful Everybody Loves Raymond, while Neil Flynn is well known for his work as The Janitor on Scrubs. Apparently, The Middle is a two-fer.
Justin Berfield managed to get a 100 episodes each of of Unhappily Ever After and Malcom in the Middle under his belt before his 18th birthday.
Her role as Megan was pretty important, right behind the titular Drake and Josh, really. She played the “evil little sister” better than anyone I’ve ever seen.
Kim Fields: “The Facts of Life” and “Living Single”
Erika Alexander: “The Cosby Show” and “Living Single”
Tisha Campbell: “Martin” and “My Wife & Kids”
Tichina Arnold: “Martin” and “Everybody Hates Chris”
Carl Anthony Payne: “The Cosby Show” and “Martin”
Fran Drescher: “The Nanny” and “Happily Divorced”
Markie Post: “Night Court” and “Hearts Afire”
Alfonso Ribiero: “Silver Spoons” and “Fresh Prince of Bel-Air.”
Keshia Knight-Pulliam: “The Cosby Show” and “Tyler Perry’s House of Payne.”
Peter Scolari: “Bosom Buddies” and “Newhart.”
Julia Duffy: “Newhart” and “Designing Women.”
Marla Gibbs: “The Jeffersons” and “227.”
Erika Alexander joined “Cosby” at the end, causing it, IMO, to “JTS”.
Wikipedia: “Following a salary dispute with NBC and production company Lorimar in 1987, Harper was fired from the series at the end of its second season. Harper sued NBC and Lorimar for breach of contract. Her claims against NBC were dismissed, but the jury found that Lorimar had wrongfully fired her and awarded her $1.4 million plus 12.5 percent of the show’s profits. The series continued without her with the explanation that her character had died off-screen.”
So it seems she made out very well from the show after all.
Cousin Pam was far less a JTS factor of The Cosby Show than the essential disappearance of 2 of 5 of the original children with Vanessa off at college (because the actress was) and Denise “in Africa” (after being fired) inexplicably leaving behind her stepchild Olivia to retreat “cute tiny child” storylines that had already been played out when Rudy was the cute tiny girl. When they aged up Winnie & Nelson to triple the cute tiny child stuff, it made it all the worse.
Cousin Pam was barely there, by comparison.
Chris Barrie - Red Dwarf, The Brittas Empire
James Garner did it better than anyone. Maverick and Rockford Files were two of the greatest shows of their time. He was also among the first actors to achieve great success in both television and film.
And I know the OP wasn’t especially keen on British sitcoms because they’re not usually as long as the US ones, but it’s worth mentioning that both Red Dwarf and The Brittas Empire ran for over a decade. Red Dwarf is still going, albeit after a fairly long hiatus.
And if we’re including cartoons, then it’s worth mentioning Jason Alexander was the voice of Duckman as well as his role on Seinfeld.
In what universe is Samantha Who considered a “successful” sitcom? It didn’t even last as long as Friends spinoff flop Joey.
8 Simple Rules is pretty borderline itself, just barely lasting long enough to squeak by into syndication, but far more of a success than Samantha.
While you could make an argument for Happy Days tanking creatively, it was still around long enough to consider it a success even after he joined. He joined in season 8, and it lasted 11. The shark jump (literally) was in season 5, three years before he joined.
Married with Children, it didn’t really tank at all, and he was there for the majority of the show’s run, seasons 4 through 11 (hmm, season 11 again… interesting.) Frankly, I’d argue the show got better after he joined.
Speaking of Happy Days, Scott Baio was both on that as well as Charles In Charge, which lasted quite a long time for a first run syndicated sitcom (though it did start on, I think, CBS)
I don’t know what Brotherly Love is, so I dunno if it was a success, but his first successful sitcom was Gimme A Break. One of his brothers joined the cast in the last season as well.
Speaking of Gimme a Break, Thelma Hopkins was both on that and Family Matters.
Marla Gibbs, her 227 costar is best known for being on the Jeffersons as Florence.
In what universe is Herman’s head successful? Jane Sibbet (Ross’s exwife on Friends) was on that show too. Hank Azaria also had a recurring role on Mad About You with his then future exwife Helen Hunt. Yeardly Smith was recurring on Dharma & Greg as Greg’s secretary.
Betty White was also on Mama’s Family for the same years as Rue. Then then the show moved from NBC to syndication and they both moved to Golden Girls. They were also on Golden Palace, but there exists no universe where that was successful (also it would be disqualified as a spin-off anyway).
Has anyone mentioned Michael J. Fox yet? Sure, he had to leave Spin City early, but I think it still should count, and of course Family Ties counts. Speaking of Spin City, maybe he wasn’t on it long enough to count for him, but Charlie Sheen was MJF’s replacement on there and of course he went on to 2.5 Men.