Nichols, but only one season.
If it weren’t for the ‘primetime’ disclaimer, Eric Braedon would be right up there. He has played ‘Victor’ on the Young and the Restless for over 4,000 episodes.
One thing that sets Boreanaz apart is how steadily he’s worked. He went right from Buffy (3 years) to Angel (5 years) to Bones (12 years) to SEAL Team, (6 years and counting) with virtually no downtime between them. There hasn’t a single year since 1997 - over a quarter of a century - in which David Boreanaz hasn’t starred in a long-running prime-time series. Can any other actor boast that kind of run?
I had thought of Lucille Ball, but it looks like there was a two-year gap between the conclusion of The Lucy-Desi Comedy Hour (the series of specials that was an irregularly-scheduled continuation of I Love Lucy) in 1960, and the beginning of The Lucy Show in 1962.
Michael Landon went from 1959 to 1989.
You are quite correct, very impressive. Is David Boreanaza about to break this record? He’s about 3 years away, though he wasn’t exactly the lead of Buffy.
Which was immediately followed by Here’s Lucy, which ran from September 1968 to March 1974.
Ed O’Neill
260 episodes of Married With Children
250 episodes of Modern Family
Yup; except for that two-year gap, she was the star of a major TV comedy series from 1951 until 1974.
Here’s one no one would have thought of: Sharon Gless
71 episodes of Switch
The final season of House Calls, replacing Lynne Redgrave
Cagney and Lacey
Not to mention long-running roles on USA’s Burn Notice and Showtime’s Queer as Folk.
Bea Arthur
Maude (All In the Family spin off, wasn’t it?)
Golden Girls
Under the name of Hans Gudegast, he also made something of a career playing Nazis and other such nasty types in Combat!, The Rat Patrol, Mission: Impossible, and Lord knows what else. Central Casting must have loved him!
His current show is ending with this next season. I’m not sure how many episodes. The previous season only had 10 episodes.
He has really had stunning success on television. Not unmatched, but even if you exclude Buffy, he was on Angel for 5 seasons, Bones for 12 seasons, and Seal Team for 7 seasons.
That is 22 straight seasons of being a lead, though I am allowing for a gap in there if there was a year or two where he wasn’t in a show.
Anyone mention Carroll O’Connor? (if so, I missed it)
206 - All in the Family
97 - Archie Bunker’s Place
146 - In the Heat of the Night
Doesn’t meet OP’s requirements of Live Action or Comedy but Ed Sullivan hosted 1048 episodes of The Ed Sullivan Show