Since I get this question so often, I thought I would put in a short page about it. All information is from the Internet Movie Database; if the person has done any other work, I am unaware of it unless I hear otherwise.
The Miller Family and Friends
George Cleveland (George "Gramps" Miller): Died July 15, 1957, of a heart attack, during the filming of Lassie's fourth season. His death was worked into the television story; see the fourth season page.
Jan Clayton (Ellen Miller): Died August 28, 1983, of cancer.
Tommy Rettig (Jeff Miller): After making a name for himself in the computer field, died February 16, 1996.
Donald Keeler (Sylvester "Porky" Brockway): Still appears in movies under his real name, Joey Vieira. He is the pizza man in Ferris Bueller's Day Off and appeared in The Patriot with Mel Gibson.
Richard Garland (Constable Clay Horton): Died May 24, 1969, from alcoholism-related problems.
Frank Ferguson (Dr. Wilson): Died September 12, 1978, of cancer.
Ray Teal (Jim Teal): Died April 2, 1976.
Florence Lake (Jenny the operator): Died April 11, 1980.
Arthur Space (Frank "Doc" Weaver, DVM): Died January 13, 1983, of cancer.
Paul Maxey (Matt Brockway): Died June 3, 1963, of a heart attack.
Marjorie Bennett (Birdie Brockway): Died June 14, 1982, of cancer.
Nolan Leary (Dan Cook, later Judge Baxter): Died December 12, 1987.
The Martin Family and Friends
Jon Provost (Timmy Martin): Sells title insurance. He still makes appearances at charity events in connection with Lassie and supports no-kill animal shelters. Visit his website.
Jon Shepodd (Paul Martin, 1957-1958): No acting credits since 1962. According to Jon Provost's website, he is now a writer in England. (There's a picture of Jon and Jon reunited on Provost's websitecheck out the "Recollections" section for some super photos, old and new.)
Cloris Leachman (Ruth Martin, 1957-1958): Continued her career, including playing Phyllis Lindstrom on The Mary Tyler Moore Show; the character was spun off onto her own eponymous series, Phyllis.
Kelly Junge Jr (Scott Richards): Apparently no acting credits since the 1950s. According to his sister, he's in the construction business now and loves to surf. (There's a photo of an adult Kelly on Jon Provost's website.)
George Chandler (Petrie Martin): Died June 19, 1985, complications from surgery.
Hugh Reilly (Paul Martin): Died July 17, 1998, of emphysema.
June Lockhart (Ruth Martin): Still appears occasionally on television and in movies; she was the school principal in 1998's movie remake of Lost in Space.
Todd Ferrell (Ralph "Boomer" Bates): Apparently no acting credits since the 1950s.
Andy Clyde (Cully Wilson): Died May 18, 1967.
Sally Bliss (Miss Hazlit): Real name Carla Belenda, apparently no acting credits since the 1950s.
Flip Mark (Flip): Apparently no acting credits since the 1960s. According to a correspondent, he's a 911 dispatcher in a Western state. (There's a photo of an adult Flip on Jon Provost's website.)
Robert Foulk (Sheriff Miller): Died February 25, 1989.
Dick Foran (Ed Washburne): Died August 10, 1979.
Stuart Randall (Al Livermore): Died June 22, 1988.
Eddy Waller (Henry Enders/Matt Krebs): Died August 20, 1977 of a stroke.
Linda Wrather (Wilhemina "Willie" Brewster): Only one acting credit besides Lassie, a role on The Lone Ranger.
Richard Correll (Steve): No acting credits past the 1950s except for 1983's Still the Beaver (Correll was a regular on Leave It to Beaver). Correspondent Teno Pollick tells me Correll has successfully produced several television series, including The Hogan Family.
Byron Foulger (Dan Porter, other parts): Died April 4, 1970.
The Ranger Years
Robert Bray (Corey Stuart): Died March 7, 1983, of a heart attack.
Clyde Howdy (Hank Whitfield): Died 1969.
Jed Allan (Scott Turner): Still doing television guest star roles; was a regular on The Bay.
Jack DeMave (Bob Ericson): Last listed appearance 2008 in movie Ladies of the House.
Mark Miranda (Neeka, later Andy Lopez): Apparently no acting credits since Lassie.
Holden Ranch Episodes
Ron Hayes (Garth Holden): Last listed appearance 2013 in TV movie Whoa!
Larry Wilcox (Dale Mitchell): Of course went on to do series CHiPs and subsequent reunion movie. Has done rare guest appearances since then, the latest being A Tale of Two Sillies.
Skip Burton (Ron Holden): Last IMDb credit As the World Turns in 1982.
Larry Pennell (Keith Holden): Died August 28, 2013. Most famous for the series Ripcord in the 1960s.
Pamelyn Ferdin (Lucy Baker): Left acting to become a nurse, now an animal rights activist. According to the IMDb, last credit 2009 as the voice of the titular character in Christmas the Horse.
Jay W. Macintosh (Elaine Baker): Last listed appearance 1994 as a guest on the series Picket Fences.
Sherry Boucher (Sue Lambert, DVM, mark 1 a.k.a. The One With the Braids): Sporadic movie and television appearances; last credit 2006's Jeremiah's Secret.
Joan Freeman (Sue Lambert, DVM, mark 2): Last listed appearance 1994 in episode of the series The Renegade.
Radames Pera (Willy): Last listed appearance 2010 in film Mojo Tango. Most famous for playing Young Caine in Kung Fu.
Joshua Albee (Mike Bishop): A few acting credits after Lassie, the last listed being a guest on the series Code Red.
Karl Swenson (Karl Burkholm): Died October 8, 1978, of a heart attack, after playing Lars Hansen on Little House on the Prairie for the first few years of the series.