Victoria Rowell was a regular on Diagnosis Murder and The Young and the Restless at the same time. Well, not every episode of the soap, of course, so I don’t know how much that qualifies.
And Seth Green did voice work on Robot Chicken, and Alex Borstein was on The Marvelous Mrs. Maisel.
Yeardley Smith and Hank Azaria were voice actors on The Simpsons while also co-starring on Herman’s Head.
Wesley Eure played Will Marshall on Land of the Lost while also playing a major character on Days of Our Lives
H. Jon Benjamin voiced the title characters on Archer and Bob’s Burgers for 12 consecutive seasons.
Nancy Walker played Rhoda’s mother, Ida Morgenstern on the sitcom Rhoda and also Mildred, the maid to Rock Hudson on McMillan and Wife at the same time over a few years. She got seven Emmy nominations between the roles. Mad magazine even pointed it out in their parody of Rhoda.
Actually, you could say she overlapped with a third, The Nancy Walker Show, in 1976
Mel Blanc is familiar all Dopers, I’m sure, because of his cartoon voice work. But he had a huge career in radio as a voice actor outside of cartoons. While he was a regular on NBC’s The Jack Benny Show he also had his own program, The Mel Blanc Show on CBS at the same time. But, what’s more:
Heather Locklear was in TJ Hooker and Dynasty at the same time.
Does McMillan and Wife really count as a series, though? It was really more like a bunch of TV movies with the same characters - the episodes were 60-90 minutes long and it wasn’t on every week and therefore had fewer episodes than series at that time usually did. In fact . it was one part of a program called the Something Mystery Movie I don’t remember if Something was the network name or the day of the week as there were multiple programs with that set-up.
If I remember right, LeVar Burton was hosting READING RAINBOW and showing up for work as Geordi LaForge on every episode of STAR TREK THE NEXT GENERATION when he started voicing Kwame on every episode of CAPTAIN PLANET.
Fred Armisen was on Saturday Night Live and Portlandia at the same time.
Also, in 2015 Seth Meyers is credited writing and doing voice acting on The Awesomes (which he created) and writing on Documentary Now! (which he co-created), while still writing and hosting Late Night with Seth Meyers (every night!).
NBC Mystery Movie (there was also a NBC Wednesday Mystery Movie series). McMillan & Wife ran in rotation with Columbo and McCloud (and, for a time, Hec Ramsey).
And, while they weren’t making 25-ish episodes a season of each of those (which was the norm back then), it was still 7 or 8 a season for each of the component series. (There’s even an industry term for this: “wheel series” or “umbrella series.”)
Despite the longer time, the word “movie” in the umbrella series title, and the fact that a particular series wasn’t on every single week, making them “movie-like,” it’s hard to argue that they weren’t truly “series” – the amount of work that the actors (and crew) would have put in to produce seven to eight 90-to-120 episodes a season certainly made them comparable to every-week TV series, as far as workload (which is, I think, kind of the gist of the OP), and the actors were playing the same characters in dozens of episodes over just a few years.
If voice work counts, Robert Ridgely played the title roles in Thundarr, the Barbarian and The New Adventures of Flash Gordon.
Although he usually played supporting characters rather than lead roles, during the 1970s and 1980s, Alan Oppenheimer often had recurring roles in several different Saturday morning cartoons at a time.
He wasn’t the star of either show, but Wayne Knight’s roles on Seinfeld and 3rd Rock from the Sun overlapped for a few years.
Apologies, this should have read “seven to eight 90-to-120-minute episodes a season”
Which reminds me, while Deidre Hall was on Days of Our Lives, she also was the lead character in Electra-Woman and Dyna Girl and Our House.
I can’t possibly guess how many soap stars also had gigs on prime time shows at the same time.
Frank Cady should count as well, appearing as Sam Drucker in 142 episodes of Green Acres and 168 episodes of Petticoat Junction. By the end of PJ he was billed right behind the Bradley daughters.
The three have done multiple shows at the same time and Always Sunny.
Glenn Howerton
A.P. Bio
Velma
Kaitlin Olson
The Mick
High Potential
Rob McElhenney
Mythic Quest
Welcome to Wrexham
The Mindy Project
Charlie Day tends to do movies rather than tv shows.
Shorter tv seasons nowadays make stuff like this easier.
If we do open this completely to voice actors, every cartoon made from 1967-72 featured at least one of six or seven very busy voice actors, like Mel Blanc, Don Messick, June Foray… Today, guys like Billy West, Mark Hamill and Seth McFarlane can be on three shows simultaneously!
Starring in multiple shows is easier these days, with series consisting of 8 episodes streaming every two years. For instance, at this very moment, Walton Goggins is starring in Fallout, White Lotus and the Righteous Gemstones.
He’s also a voice actor in Invincible