Heckart was born Anna Eileen Herbert in Columbus, Ohio, the daughter of Esther Stark, who wed Leo Herbert (not the child's father) at her own mother's insistence so her child would not be born with the stigma of illegitimacy. The child was soon after legally adopted by her maternal grandmother's wealthy second husband, J.W. Heckart, the surname by which she would be known her entire life. She had two stepsisters, Anne and Marilyn. She graduated from The Ohio State University with a B.A. in drama.
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, And Things That Go Bump in the Night, Barefoot in the Park, Butterflies Are Free, You Know I Can't Hear You When the Water's Running, and The Cemetery Club.
In 2000, at age 81, she appeared off-Broadway in Kenneth Lonergan's The Waverly Gallery, receiving more awards for a single performance in a single season than any actress in theatre history, including the Drama Desk Award, the Lucille Lortel Award, the Drama League Award and the Outer Critics Circle Award. That same year, she was inducted into the Theatre Hall of Fame and received an honorary Tony Award for lifetime achievement.
Other awards include the 1953 Theatre World Award for Picnic. Her nominations include Tony Award nominations for Butterflies Are Free, Invitation to a March, and The Dark at the Top of the Stairs. She was granted three honorary doctorates by Sacred Heart University, Niagara University and Ohio State University.
Heckart won the Academy Award 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, both of which were roles Heckart had originated on Broadway. She also appeared as a Vietnam War widow in the Clint Eastwood film, Heartbreak Ridge. She played Diane Keaton's meddling mother in the 1996 comedy film The First Wives Club.
On television, Heckart had starring roles in The Five Mrs. Buchanans, Out of the Blue, Partners in Crime, and Backstairs at the White House (Emmy nomination as Eleanor Roosevelt). In 1994, she won an Emmy Award for Outstanding Guest Actress in a Comedy Series for her appearance as Rose Stein on Love & War. Her other guest spots included The Fugitive (where she appeared in three episodes as a nun, "Sister Veronica"), The Mary Tyler Moore Show (two Emmy nominations as journalist Flo Meredith, a role she carried over to a guest appearance on MTM's spinoff Lou Grant), Love Story, Rhoda, Alice, Murder One, Hawaii Five-O, Gunsmoke, Cybill, The Cosby Show (one Emmy nomination as Mrs. Hickson), and many others.
Heckart played two unrelated characters on the daytime soap opera One Life to Live. During the 1980s, she played Ruth Perkins, the mother of Allison Perkins, who had kidnapped the newborn baby of heroine Vicky Lord Buchanan under orders from phony evangelist and mastermind criminal Mitch Laurence. During the early 1990s, she played the role of Wilma Bern, mother of upstate Pennsylvania mob boss Carlo Hesser and his meek twin, Mortimer Bern. She appeared in the 1954 NBC legal drama Justice, based on case files of New York's Legal Aid Society. She appeared in an episode of the NBC medical drama about psychiatry, The Eleventh Hour, "There Should Be an Outfit Called 'Families Anonymous!'" (1963).
Heckart was married to John Harrison Yankee, Jr. for 55 years from 1942 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.
On December 31, 2001, Heckart died of lung cancer at her home in Norwalk, Connecticut at the age of 82. She was survived by her three children and her two stepsisters.
The Eileen Heckart Collection was established at Ohio State University's Jerome Lawrence and Robert E. Lee Theatre Research Institute, with her notes, copies of scripts, and personal papers. In 2005, the Eileen Heckart Drama for Seniors Competition was established in her memory by Ohio State's Department of Theatre. Her sons also established a scholarship at Ohio State in her name.
Heckart has a Star on the Hollywood Walk of Fame at 6162 Hollywood Blvd.