On October 28, 1958, Rettig guest-starred in the episode "The Ghost" of the ABC/Warner Brothers western series Sugarfoot with Will Hutchins in the title role. In the series installment, Rettig played Steve Carter, a troubled youth whom Sugarfoot is taking to Missouri to collect an inheritance. Rettig also sang the popular ballad "The Streets of Laredo" in the episode.

Rettig graduated in 1959 from University High School in Los Angeles. 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 western series The Man from Blackhawk, starring Robert Rockwell as a roving insurance investigator. Actress Amanda Randolph was cast in the same episode as Auntie Cotton.

In the 1962 episode "Davy's Friends" of the syndicated series Death Valley Days, Rettig played Joel Walter Robison, a fighter for Texas independence. In the storyline, Robison, called a "friend" of Davy Crockett, is sent on a diversion but quickly shows his military ability and is made a first lieutenant by Sam Houston. Stephen Chase played Sam Houston, and Russell Johnson was cast as Sergeant Tate in this episode.

From 1964 to 1965, Rettig co-starred with another former child actor, Tony Dow, in the ABC television soap opera for teens Never Too Young. With the group "The TR-4", he recorded the song by that title on the Velvet Tone label. While he was the TR-4's co-manager, he did not sing with them. Rettig only co-wrote the song in hopes that the TV soap would use it as the series' theme. Producers of Never Too Young, however, chose not to use it. Rettig was subsequently cast as Frank in 1965 episode "The Firebrand" on the NBC education drama series Mr. Novak, which starred James Franciscus.

For the last 15 years of his life, Rettig was a well-known database programmer, author, and expert. He was an early employee of Ashton-Tate and specialized in (sequentially) dBASE, Clipper, FoxBASE and finally FoxPro. Rettig moved to Marina del Rey in the late 1980s.

Rettig made a guest appearance as a grown-up Jeff Miller in an episode of the later television series The New Lassie, with Jon Provost, which aired on October 25, 1991. The updated series featured appearances from two other Lassie veterans: Roddy McDowall, who in 1943 had starred in Lassie Come Home, the famous dog's first feature-length film, and June Lockhart, who had starred in the 1945 sequel Son of Lassie. Lockhart had also co-starred on the earlier television series, portraying Timmy's mother in the years after Rettig and Actress Jan Clayton left the show.

Rettig died in 1996 at age 54 of a heart attack. Attendees to his memorial Service in Marina del Rey, California, included Roger Clinton, Jr., the half-brother of then U.S. President Bill Clinton; Lassie; Microsoft dignitaries; and several former child stars, who were featured in a photo spread in The National Enquirer.

As an adult, Rettig preferred to be called "Tom." He found the transition from child star to adult difficult, and he had several well-publicized legal entanglements relating to illegal recreational drugs (a conviction for growing marijuana on his farm, and a cocaine possession charge of which he was exonerated). Some years after he left acting, he became a motivational speaker, which—through work on computer mailing lists—led to involvement in the early days of personal computers.