While the OP never gave a reason for the cutoff, nor needs to, I posted my own reason for taking this as a suitable cutoff here.
Shep Houghton turned one hundred last month – it was in all the papers – after making his movie debut in the '20s and getting small parts in big movies in the decades since: he’s one of the Southerners milling around in Gone With The Wind, he’s a nightclub patron in The Big Sleep, he’s a Roman soldier if Claudette Colbert is playing Cleopatra, he’s a French soldier if Ingrid Bergman is playing Joan of Arc, you name it.
And he acted in a number of Dragnet episodes before the OP’s cut-off date, followed by acting in a number of Ozzie & Harriet episodes soon thereafter.
In the '60s, Harry Landers spent five seasons as Dr. Ted Hoffman of Ben Casey fame; before that, he earned dozens of pre-'58 credits on everything from Perry Mason to Have Gun - Will Travel to Alfred Hitchcock Presents to Playhouse 90 and so on.
Jane Powell was plenty busy on television pre-1958: acting on Goodyear Theatre and Alcoa Theatre, making appearances on The Ed Sullivan Show, playing mystery guest on What’s My Line – even doing Producers’ Showcase with a young Hal Linden.
From '58 to '64, Paul Petersen did hundreds of Donna Reed Show episodes; he then spent the rest of the decade acting on The Flying Nun and Mannix and so on, followed by yet more roles in the '70s and '80s and '90s and '00s and '10s – and branching out as a writer/director/producer to boot – all after, y’know, qualifying for this thread on The George Sanders Mystery Theater in '57 and The Ford Television Theatre in '56.
Roberta Shore, who I’m pretty sure wasn’t just Robert Shaw in drag, was acting on Playhouse 90 with Eddie Cantor and Peter Lorre back in '56 – after getting work on The Fireside Theater with Jane Wyman, and appearing on The Pinky Lee Show to boot. (In '60 and '61, she was Joyce on Ozzie & Harriet; in '61 and '62, Henrietta on The Bob Cummings Show; and from '62 to '65, Betsy on The Virginian.)
Ann Gillis did dozens of movies in the '30s and '40s – she was Becky Thatcher in the classic version of Tom Sawyer back in '38, voiced Faline in Bambi, and as per wiki is the last surviving member of the Beau Geste cast – and in the early '50s, she was a TV actress by way of Out There and Studio One In Hollywood.
Oh, and in the '60s, you could see her in 2001: A Space Odyssey.
Now ninety-nine, WWII veteran Jan Merlin is still with us sixty years after he was getting steady work as Roger Manning on Tom Corbett, Space Ranger.
After that '51-to-'54 run he played Oliver Wendell Holmes Jr in a 1955 TV movie, and then acted on a dozen shows before the end of '57, after which he spent '58 and '59 as a regular on The Rough Riders. And he kept working in the '60s – Perry Mason, Mission: Impossible, Bonanza, and et cetera – before winning an Emmy in the '70s, and then getting right back to work in the '80s whenever the gang on The A-Team or Riptide or Dallas or whatever had something interesting for him to do.
Earl Cameron was acting on Chevron Theatre in '52, and after doing a couple of TV movies got work on a few TV shows before landing his first recurring role in '57.
Most recent IMDB credit was back in October, at the age of ninety-six.
After making his Broadway debut in the late '50s – in The Music Man, alongside the inimitable Robert Preston – Eddie Hodges became one of the few people to act on both Gunsmoke and LAW & ORDER, taking the time in between to do an Elvis movie in the '60s and a kung-fu movie in the '70s and et cetera.
Anyhow, back in '59 Hodges memorably sang “High Hopes” with Frank Sinatra on the big screen in A Hole In The Head; the year before that, he was acting on Omnibus; and in the years before that, The Jackie Gleason Show and Name That Tune.
Yes, the episode with John Glenn. (Who’s also still with us, come to think of it.)
Lydia Reed, who acted in High Society with Bing Crosby and Grace Kelly – as well as The Seven Little Foys, with Bob Hope and Jimmy Cagney – racked up credits on over a dozen television shows before she started her '57-'63 run on The Real McCoys, all after making her debut in a TV movie back in 1952.
Over the last decade, Patty McCormack went from a recurring role on The Sopranos to a recurring role on Hart of Dixie, stopping off to do Frost/Nixon in between.
In '57, she was acting on episodes of Playhouse 90; before that, she was (a) getting work on Kraft Theatre and Matinee Theatre – and The Revlon Mirror Theater, and General Electric Theater, and a dozen others besides – and (b) earning herself an Oscar nomination by reprising her most famous Broadway role for the big screen.
Since someone just mentioned Stan Freberg in another thread, I’ll add that in '54 and '55 he performed as a comedian on The Colgate Comedy Hour before doing likewise on half-a-dozen episodes of The NBC Comedy Hour in '56 – not to mention doing a little acting on The Eddie Cantor Comedy Theater along the way.
(All after his first Emmy nomination back in '51, you understand; the guy started working on television in the '40s, long before he was the voice of Pete Puma.)
Sadly, Garner just died. 
Sally Ann Howes turned 84 today. Credits go back to 1943 with first TV listing from 1950. Hasn’t done TV/movies in a long while but was performing on stage until fairly recently.
Gina Lollobrigida was appearing on The Jimmy Durante Show and The Ed Sullivan Show in '54 and '55 respectively, after making her TV debut on What’s My Line with Steve Allen decades before doing all those Falcon Crest episodes in the '80s.
Lydia Clarke was married to Charlton Heston for sixty-four years and acted alongside him on The Philco-Goodyear Playhouse back in '53, after doing episode after episode of Studio One In Hollywood. (Before that, she was earning the Theatre World Award on Broadway – oh, and appearing in The Greatest Show On Earth, of course.)
Louise Fletcher turns 80 today. Unfortunately, her first IMDb listing is for 1958. Interestingly enough, Wikipedia says that her appearance on Maverick in 1959 was the highest rated episode of the late great James Garner’s show.
I’m hoping someone will turn up a pre-1958 credit for her.
For thread-conformance: One of her co-stars in a Playhouse 90 episode was Isabel Cooley who started on TV in 1957 and continued to 1990s. Apparently still alive.
Another co-star in one of her early credits is the thread-popular Rance Howard.
I think we can count Connie Francis, who appeared on a number of TV shows before 1958 (when she was on all those Dick Clark Show episodes) and 1959 (when she was acting on The United States Steel Hour and singing on The Ed Sullivan Show).
Clifton James was the fat Southern sheriff in Roger Moore’s 007 movies, and the fat Southern sheriff in SILVER STREAK and SUPERMAN II; he was also a fat Southern sheriff on THE FALL GUY and THE A-TEAM and THE DUKES OF HAZZARD; and he was, of course, a state trooper on The Phil Silvers Show back in 1957, after making his Broadway debut as – and I’m quoting, here – “A Cop”.
Movie in the works for later this year, incidentally.