Heckart began her Broadway career as the assistant stage manager and an understudy for The Voice of the Turtle in 1943. Her many credits include Picnic, The Bad Seed, A View from the Bridge, A Memory of Two Mondays, The Dark at the Top of the Stairs, A Family Affair, Barefoot in the Park, Butterflies Are Free, and The Cemetery Club. Her awards include a special 2000 Tony Award for Excellence in the Theatre, a 2000 Drama Desk Award for Outstanding Actress in a Play for the off-Broadway The Waverly Gallery, and the 1953 Theatre World Award for Picnic. Her nominations include a 1996 Drama Desk Award for Northeast Local and Tony nods for Butterflies Are Free, Invitation to a March, and The Dark at the Top of the Stairs.

Heckart won the Oscar for Best Supporting Actress for her work in the 1972 movie adaptation of Butterflies Are Free and was nominated in 1956 for her performance as the bereaved, besotted Mrs. Daigle in The Bad Seed. She also appeared as a Vietnam War widow with Clint Eastwood in Heartbreak Ridge.

Heckart was familiar to TV audiences with starring roles in The Five Mrs. Buchanans, Annie McGuire, Out of the Blue, Trauma Center, Partners in Crime, Backstairs at the White House (as Eleanor Roosevelt), and guest spots on The Mary Tyler Moore Show (as journalist Flo Meredith, a role she carried over to a guest appearance on MTM's spinoff Lou Grant), Rhoda, Murder One, Cybill and many other shows.

Heckart was married to John Harrison Yankee Jr. from 1944 until his death in 1997. Her son Luke Yankee is the author of Just Outside the Spotlight: Growing Up with Eileen Heckart (ISBN 0-8230-7888-4), published by Back Stage Books in 2006.

A heavy smoker for much of her life, Heckart died of cancer at her home in Norwalk, Connecticut at the age of 82. She also had lived in Stamford, Connecticut.^[1] She was survived by 3 children and 2 stepsisters.