[3] His character was a young farm boy who lived with his widowed mother, Ellen (Jan Clayton 1917–1983), grandfather (George Cleveland, 1885–1957), and his beloved collie, Lassie.

In addition to his famous role as Jeff Miller in the Lassie television series on the CBS network, Rettig also appeared in 17  other feature films, including So Big (1953), The 5,000 Fingers of Dr. T. (1953), written by famed children's books author Dr. Seuss, and following in River of No Return (1954) with Marilyn Monroe and Robert Mitchum.

He reportedly received no residual payments from his several years work in the Lassie series, even though it was later very popular in syndication re-runs, widely shown under the changed title Jeff's Collie.

In the same year at the age of 18, he was cast as Pierre in the episode "The Ghost of Lafitte", set in New Orleans, of the ABC short-lived western series The Man from Blackhawk (1959–1960), starring Robert Rockwell as a roving insurance investigator.

Hawks manages to escape, but the wagonmaster Major Seth Adams (Ward Bond) makes the difficult decision not to attempt Billy's rescue, lest risking the entire wagon train to be vulnerable to attack.

From 1965 to 1966, Rettig co-starred with another former child actor, Tony Dow (famous from Leave it to Beaver), in the ABC television soap opera for teens, Never Too Young.

[9] Rettig was subsequently cast as Frank in the 1965 episode "The Firebrand", of the NBC education / high school drama series Mr. Novak (1963-1965), which starred James Franciscus.

He found the transition from child star to adult to be difficult, and he had several well-publicized legal entanglements relating to illegal recreational drugs, a conviction for growing marijuana on his farm in 1972, and a cocaine possession charge in 1976, of which he was exonerated.