[3] Thomas later starred as Bill Denbrough in the 1990 television miniseries adaptation of Stephen King's epic horror novel It, and played Special Agent Frank Gaad on FX's spy thriller series The Americans.

[4] In 1959, he appeared in the Hallmark Hall of Fame NBC television presentation of Ibsen's A Doll's House with Julie Harris, Christopher Plummer, and Hume Cronyn.

[9] He then began acting in daytime TV, appearing in soap operas such as The Edge of Night (as Ben Schultz, 1961), A Flame in the Wind and As the World Turns (as Tom Hughes, 1966–67) which were broadcast from his native Manhattan.

Thomas's first major film roles were in the auto racing drama Winning (1969) with Paul Newman[10] and the coming-of-age story Last Summer (also 1969) with Bruce Davison and Barbara Hershey.

[11] In 1971 Thomas appeared in The Todd Killings, a psychological thriller released by National General Pictures, directed by Barry Shear and co-starring Robert F. Lyons, Belinda Montgomery and Barbara Bel Geddes, based on the true crimes of serial killer Charles Schmid.

[18] In other TV films, he played Col. Warner's younger son Jim in Roots: The Next Generations (the 1979 sequel to 1977's Roots),[19] the title role in the biopic Living Proof: The Hank Williams Jr. Story in 1983,[20] Will Mossup in CBS's Hobson's Choice in 1983,[21] Henry Durie in The Master of Ballantrae for Hallmark Hall of Fame,[22] Martin Campbell in Final Jeopardy,[23] and the adult Bill Denbrough in the 1990 television mini-series It, adapted from Stephen King's horror novel.

[25] In the same year, he appeared as Shad (the young farmer entrusted to employ mercenaries to save his planet from Sador and his invading forces) in Battle Beyond the Stars.

[27] In 1990, he joined with Nathan Lane at the Mark Taper Forum[28] in Los Angeles for Terrence McNally's The Lisbon Traviata in the role of Stephan.

In 1993, he played the title role in a Shakespeare Theater stage production of Richard II in Washington, D.C.[29] Thomas starred in the ABC TV movie Death in Small Doses, directed by Sondra Locke.

[30] Thomas appeared in a quartet of performances at the Hartford Stage in Connecticut including Hamlet (1987),[31] Peer Gynt (1989), Richard III (1994) and Tiny Alice (1996).

In January 2022, Thomas portrayed Wendy Byrde's estranged father, Nathan Davis, in three episodes of season 4 of the Netflix series Ozark.

Starting in April 2022, Thomas starred as Atticus Finch in a National Broadway tour of a stage production of Harper Lee's novel To Kill a Mockingbird.