[12] In April 1945, Montealegre made her first New York acting appearance in the English-language premiere of Federico Garcia Lorca's If Five Years Pass at the Provincetown Playhouse.

[13] In 1950, she was an understudy to Leora Dana in Samuel A. Taylor's The Happy Time on Broadway, starring Eva Gabor and Montealegre's then-lover Richard Hart.

[18] Montealegre returned to the Broadway stage in 1967 to play Birdie Hubbard in Lillian Hellman's The Little Foxes directed by family friend Mike Nichols.

[23] In 1950, she appeared in the leading role of Nora Helmer in Henrik Ibsen's A Doll's House, with John Newland as Krogstad and Theodore Newton as Thorvald.

[25] She made her first appearance on the CBS Television Network's Studio One in the psychological thriller Flowers from a Stranger, which aired on May 25, 1949, with actor Yul Brynner.

[26] She acted in eleven Studio One teleplays between 1949 and 1956, including Of Human Bondage (aired November 21, 1949),[27] based on W. Somerset Maugham's novel in which Montealegre played Mildred opposite Charlton Heston as Philip Carey.

3: Kaddish with Montealegre in mind, and she narrated its American premiere with soprano Jennie Tourel and Charles Munch conducting the Boston Symphony Orchestra on January 31, 1964.

[34] In 1963, Montealegre became the first chair of the Women's Division of the New York Civil Liberties Union, where her efforts focused on educational programs and fundraising events.

[37] Initiated on Mother's Day of 1967, volunteers mailed postcards to President Lyndon B. Johnson and members of Congress with the message that "War is not healthy for children and other living things.

Montealegre sharply condemned the response in a letter to The New York Times, writing: "The frivolous way in which it was reported as a 'fashionable' event is ... offensive to all people who are committed to humanitarian principles of justice.

[45] The report criticized the theory behind and practice of the New York State Board of Parole and recommended its abolishment as an institution, provided an alternative could be found.

"[53] Montealegre maintained close friendships with many artists and intellectuals, including Marc Blitzstein, Lillian Hellman, Jennie Tourel, Richard Avedon, Martha Gellhorn, Stephen Sondheim, Cynthia O'Neal, and Michael "Mendy" Wager, with whom she kept regular correspondence.

[56][57] Montealegre was hailed by the Guild of Professional Beauticians as one of "The Ten Best Coiffured Women of 1964" along with well-known female contemporaries such as Eileen Ford, Anne Klein, and Debbie Reynolds.

"[59] In her spare time, Montealegre took up painting and studied with postwar artists Daniel Schwartz and Jane Wilson, wife of John Gruen.