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.
Richard Garland (Constable Clay Horton): Died May 24, 1969.
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 was recently remarried. He still makes appearances at charity events in connection with Lassie. 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.
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.
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.
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): Appears in the soap opera Port Charles.
Jack DeMave (Bob Ericson): Last listed appearance 1993 in movie The Man Without a Face.
Mark Miranda (Neeka, later Andy Lopez): Apparently no acting credits since the late 1960s.
Holden Ranch Episodes
Ron Hayes (Garth Holden): Last listed appearance 1988 in TV movie Dead Solid Perfect.
Larry Wilcox (Dale Mitchell): Of course went on to do series CHiPs and subsequent reunion movie.
Skip Burton (Ron Holden): Apparently no acting credits since the 1970s.
Larry Pennell (Keith Holden): Appearing in movies.
Pamelyn Ferdin (Lucy Baker): Left acting to become a nurse, now an animal rights activist. According to the IMDb, recently a voice in the series Detention.
Jay W. Macintosh (Elaine Baker): Last listed appearance 1994 in movie The Haunting of Seacliff Inn.
Sherry Boucher (Sue Lambert, DVM, mark 1): Has a credit in a new movie, Solange. Before that, no acting credits since the 1970s.
Joan Freeman (Sue Lambert, DVM, mark 2): Last listed appearance 1991 in episode of the series The Commish.
Radames Pera (Willy): Last listed appearance 1986 in episode of the series Starman.
Joshua Albee (Mike Bishop): Apparently no acting credits since the 1970s.
Karl Swenson (Karl Burkholm): Died October 8, 1978, of a heart attack.