Caldwell made her Broadway debut in 1965 when she replaced an ailing Anne Bancroft in The Devils. The following year she starred opposite Kate Reid and Margaret Leighton in Tennessee Williams' Slapstick Tragedy, an evening of two one-act plays.
The production opened at the Longacre Theatre Feb. 22, 1966, to poor reviews. The New York Times' Stanley Kauffmann wrote: "The sad news from the Longacre is that there really is no news. Mr. Williams has neither grown nor changed."
Despite the negative critical response and the show's brief run of just seven performances, Caldwell was recognized for her work and received the 1966 Tony for Best Featured Actress in a Play.
Caldwell next appeared on Broadway in The Prime of Miss Jean Brodie, Jay Presson Allen's adaptation of the Muriel Spark novel. She starred as the title character in a cast that included Douglass Watson, Lennox Milne and Roy Cooper.
The play opened Jan. 16, 1968, at the Helen Hayes Theatre, where it ran for 379 performances. Caldwell was lauded for her portrayal of schoolteacher Miss Jean Brodie; New York Times critic Clive Barnes commended her ability to "demand and get the audience's sympathy" and claimed she was "surrounded with an air of ineffable confidence."
Fourteen years later Caldwell appeared in the title role in a revival of Euripides' Medea, freely adapted by Robinson Jeffers. The cast also starred Judith Anderson as the Nurse and Mitchell Ryan as Jason.
Directed by Robert Whitehead, Medea opened May 2, 1982, at the Cort Theatre. Critic Frank Rich raved about Caldwell's performance in his review for the New York Times. "Euripides demands an intense psychological realism from actors," he wrote, "and that is what Miss Caldwell has bestowed on her marathon role."
She won her third Tony Award, the 1982 Tony for Best Actress in a Play, for her performance.
In 1995 Caldwell took on the role of Maria Callas in the Broadway premiere of Terrence McNally's Master Class. The cast also featured Audra McDonald as Sharon, Karen Kay Cody as Sophie, David Loud as Manny and Jay Hunter Morris as Tony.
The production opened Nov. 5, 1995, at the John Golden Theatre and was well received by critics. In his review for the New York Times, Vincent Canby wrote: "For Ms. Caldwell, the role of the preeminent diva of the second half of this century allows her to give what could be one of the funniest, most moving and gaudiest performances of this season and, perhaps, of her career."