He won a Theatre World Award for his role as Corporal Billy Jester, and married one of the show's producers, Cynthia Baer, in 1960; they divorced in 1971.

McMartin later starred in the original Broadway production of Sondheim's Follies opposite Alexis Smith in 1971 as Benjamin Stone, introducing the ballad "The Road You Didn't Take".

[10][11][12] Other Broadway roles include the Narrator in Happy New Year, Ben in A Little Family Business (adapted by Jay Presson Allen, 1982),[13] Donner in Tom Stoppard's Artist Descending a Staircase, Cap'n Andy in Kern and Hammerstein's Show Boat (1994),[14] and Uncle Willie in Cole Porter's High Society (1998).

[15] He also had a role as the American Revolutionary naval hero John Paul Jones in the unsuccessful Loesser/Spewack musical Pleasures and Palaces, which closed in Detroit.

[17] McMartin was a leading member of the New Phoenix Repertory Company during its three Broadway seasons in the early 1970s, appearing onstage in Eugene O'Neill's The Great God Brown (opposite Katherine Helmond), Molière's Dom Juan, and Luigi Pirandello's The Rules of the Game.

He played Thomas Jefferson in the original cast of John Guare's A Free Man of Color at Lincoln Center (2010–11), and Elisha Whitney in the 2011 Broadway revival of Anything Goes, opposite Jessica Walter.

[19] McMartin's film roles include the foreign editor in All the President's Men (1976), a senator in Brubaker (1980), a political advisor in Blow Out (1981), and millionaire Mr. Forrester in Legal Eagles (1986).