James Noble, who you maybe remember as the Governor on Benson, was acting on TV before the cut-off date – as was Inga Swenson, who played Gretchen.
The OP’s choice of cutoff is turning out to be a surpisingly interesting choice for reasons other than I’ve given before.
I keep coming across a lot of older, active actors whose IMDb credits start in 1958 or 1959. E.g., Lisa Lu.
I don’t recall anything big changing around that time that would cause a lot of actors to enter the business. Could have become “more respectable” to be a TV actor rather than a movie actor around then. (As well as better chances of finding work.) The old studio system was breaking down around then, but that stretched out for years.
It also could be merely a bias caused by credits not being easily found for earlier shows.
Anyway. Shani Wallis started in 1956 and was on a soap as recently as 2004.
Another 1956 starter was Steven Marlo. But I am less sure if he is still alive.
Gloria Henry didn’t start playing Dennis the Menace’s mom until '59, but in '57 you could see her on Tales of Wells Fargo and Perry Mason and Father Knows Best and et cetera, sure as she had a recurring role on My Little Margie years before that, and sure as she was a regular on The Files of Jeffrey Jones before that, after doing more than a dozen movies in the '40s.
Her most recent acting credit? Parks and Recreation, back in 2012.
Appearing even more recently on Parks and Recreation: two-time Emmy winner Bonnie Bartlett, who more than a quarter of a century ago was playing the mom of Arnold Schwarzenegger and Danny DeVito in Twins, and more than a quarter of a century before that was acting on Love of Life in '55 and '56 and '57.
Hey, you know who else was on Love of Life in '57?
(Same year he was a big-screen leading man in Johnny Gunman, as it happens?)
Martin E. Brooks of Six Million Dollar Man and Bionic Woman fame, who made his TV debut back in 1951 – on Sure As Fate – before acting on Suspense in '52 and '53 and '54 and otherwise making the rounds on everything from Armstrong Circle Theatre to Climax! to Studio One In Hollywood to The Philco-Goodyear Television Playhouse.
One more Love of Life alum: Audrey Peters, who joined the show in 1959 and kept at it for over two decades before moving on to All My Children.
(And, yes, she was acting on TV before the deadline: in 1956, on The Life of Riley. Her most recent IMDB credit: half a century later, on Guiding Light.)
Yet another Love of Life-r: Katherine “Scottie” MacGregor, who started acting on the show in '56 – after her big-screen debut in On The Waterfront – decades before her most memorable role, as Harriet Oleson on Little House On The Prairie.
After half-a-dozen movie roles, Lori Nelson made her TV debut on It’s A Great Life and did episodes of Climax! in '55 and '56 before becoming a castmember (alongside young Barbara Eden) on How To Marry A Millionaire in '57 and '58; she then made the rounds on all those westerns – Tales of Wells Fargo, Sugarfoot, The Texan, Laramie, Whispering Smith, Wanted: Dead Or Alive, and so on – before eventually returning to big-screen work in the '90s and '00s.
Also, she was famously Tab Hunter’s beard. And was engaged to Burt Reynolds!
But I digress. Anyhow, her other pre-cutoff-date appearances ran the small-screen gamut from The Bob Hope Show to a TV movie with Claude Rains and Jim Backus.
Peter Vaughan made his debut in a TV movie back in '54 before getting work on series after series prior to the cut-off date; he’s now in his nineties, busily plugging away at his recurring role as Maester Aemon on Game of Thrones.
After making television history on the first episode of The Ed Sullivan Show in the 1940s, Monica Lewis sang on The Milton Berle Show and The Jackie Gleason Show when she wasn’t busy acting on shows like Studio 57 and Make Room For Daddy; other work before the cut-off date include a TV movie featuring Hope and Crosby, and also Appointment With Adventure alongside a young Paul Newman.
After directing Redford and Fonda in Please Don’t Eat The Daisies, Gene Saks achieved film immortality by directing Lemmon and Matthau as The Odd Couple before earning Tony Award after Tony Award directing folks on Broadway – but before all of that, he was a struggling actor on Omnibus in '54 and Ponds Theater in '55 and Playwrights '56 in, well, '56 – plus various other pre-'57 small-screen roles.
Wright King acted on Captain Video and his Video Rangers back in '49 before reprising his Broadway role on the big screen in Streetcar Named Desire; he then appeared on dozens of pre-'58 TV shows – even if you skip half-a-dozen of the pre-'58 TV westerns he worked on – but the key is that he stuck with sci-fi back then, starring in all those Johnny Jupiter episodes back in '53 and '54 long before moving from Twilight Zone roles to playing Doctor Galen on Planet of the Apes.
Gordon Sterne was acting on television in '56, and still at it in '66 and '76, and was up on the big screen in Highlander in '86 before heading back to the small screen in '96 for Ted Danson’s TV movie of Gulliver’s Travels, and after '06 spent '07 and '08 and '09 on The Tudors, and et cetera.
Anyhow, he made his debut on I Spy. (No, not that I Spy; the one back in '55.)
Back in '49, Marjorie Stapp was a big-screen Queen Guinevere with George Reeves as the title character in The Adventures of Sir Galahad; from 1950 to 1953, she acted in half-a-dozen Fireside Theatre episodes, followed by getting work on everything from Dragnet to Ozzie & Harriet to The George Burns And Gracie Allen Show and et cetera before the cut-off date, along with plenty of westerns: Death Valley Days, Cheyenne, 26 Men, you get the idea.
She later went from The Brady Bunch to Quantum Leap, but it’s her movie roles that can’t help but catch the eye on IMDB; she’s “Woman Getting Dressed”, she’s “Blonde”, she’s “Tall Blonde”, she’s “Tall Blonde Saloon Dancer”, she’s “Min, The Tall Blonde”, she’s “Murdered Blonde”: that’s practically a whole story right there!
Monty Hall hosted TV show after TV show after TV show before the deadline, and of course kept plugging away on television since.
Fifty years before he was the old man with a hansom cab on How I Met Your Mother, young Buck Kartalian was up on the big screen acting on Mister Roberts in '55, and before that he was getting small-screen work on Robert Montgomery Presents, and before that he was on Broadway, and before that he was serving in WWII.
His IMDB page is pretty wide-ranging: The Outlaw Josey Wales with Clint Eastwood; The Rock with Sean Connery; and Batman and The Munsters and everything else from Cool Hand Luke to Gymkata; an episode of Friends, an episode of Just Shoot Me, an episode of Felicity, an episode of Curb Your Enthusiasm, you name it.
(“You know the saying: ‘Human See, Human Do’.”)
Anne Whitfield racked up half-a-dozen TV credits before the end of '57, taking time along the way to appear on the big screen with Bing Crosby in White Christmas; in '58, she turned twenty before getting work on The Donna Reed Show; in '59, she turned twenty-one and got work on the Mark Twain episode of Bonanza; and et cetera in the '60s and '70s – and '80s, at which point you could see her in a supporting role to young Helen Hunt as the Quarterback Princess when she wasn’t busy acting alongside a post-The Right Stuff Ed Harris in The Last Innocent Man.
For what it’s worth, Della Reese was singing on The Ed Sullivan Show in '61, and '60, and '59, and et cetera back through to '58 and even '57 – the year she appeared on The Big Record with everyone from Liberace to Jimmy Dean – after making her TV debut as a guest vocalist on Stage Show way back in '56.
Wiki says radio personality Don Kennedy (a) started hosting a TV show back in '56, and went on to get voice work on The Brak Show and Aqua Teen Hunger Force before retiring in September, and (b) shouldn’t be – but often is – confused with the other Don Kennedy, also born in 1920; their info and credits are “intertwined” on IMDB.
So one of 'em was acting on The Adventures of Ellery Queen in 1950 – and maybe it’s the same guy who acted on Fireside Theatre in '51, and in a Joan of Arc TV movie with EG Marshall in '52, and then The Pepsi-Cola Playhouse in '53, and so on. Maybe it’s the same one who acted last year in Your Pretty Face Is Going To Hell.
The point is, you can count at least one Don Kennedy, and probably two, for purposes of this thread; one was in a bunch of TV westerns back when; possibly both were; one is credited on episodes of The Monkees; for all I know, they took turns; I wish I knew which was which, but one way or another it seems kosher.
Before he was serenading a beauty-school drop-out, Frankie Avalon did all those beach movies in the 1960s, and before that he spent the late '50s making the rounds on television: singing on The Ed Sullivan Show after the cut-off date, playing trumpet on The Jackie Gleason Show before it, and et cetera for The Milton Berle Show and The Dick Clark Show and so on and so on.