Other notable film roles include West Side Story (1961), That Touch of Mink (1962), Move Over, Darling (1963), Freaky Friday (1976), National Lampoon's European Vacation (1985), Teen Wolf Too (1987) and The Frighteners (1996).

[2][3][4] Astin started his performing career in theater, making his first Broadway appearance as an understudy in Major Barbara (1954) and also did voice-over work for commercials.

He appeared as "Ready-Money Matt" in the long-running off-Broadway production of Threepenny Opera (which began in 1954), starring Lotte Lenya.

On April 7, 1961 he played the role of Charlie in the Twilight Zone episode, "A Hundred Yards Over the Rim" which starred Cliff Robertson.

I'm Dickens, He's Fenster received critical raves, but was against two high-rated shows, Sing Along with Mitch on NBC and Route 66 on CBS.

With the death of Lisa Loring, who played Wednesday, in January 2023, Astin is the last surviving cast member of The Addams Family.

He had a recurring role on the sitcom Night Court as Buddy, eccentric former mental patient and the father of lead character Harry Stone, who often ended his conversations with a big smile and the phrase,"...but I'm feeling MUCH better now!"

He also guest starred on numerous television series, including appearances on Duckman, Homeboys in Outer Space, Jack Palance's ABC circus drama The Greatest Show on Earth, and a 1967 episode of Gunsmoke as Festus Haggen's cousin Henry.

Astin received an Academy Award nomination for Prelude, a short film that he wrote, produced, and directed.

Astin served for four years on the board of directors of the Writers Guild of America, and has been active in community affairs in Los Angeles and Santa Monica.

He also has toured the one-man play Edgar Allan Poe: Once Upon a Midnight, written by Paul Day Clemens and Ron Magid.