Heckart has worked continuously in all three media since hitting New York in 1943. After understudying many roles on Broadway, playing small parts, and appearing with stage companies out of NYC during the 1940s, Heckart became a Broadway star by creating the role of Rosemary Sidney in "Picnic", for which she won an Outer Critics Circle Award. The next year, she created the role of Mrs. Daigle in the original stage production of "The Bad Seed". Heckart was brought to Hollywood by Warner Brothers to recreate the role in the 1956 film version for which she earned her first Oscar nomination as Best Supporting Actress. That same year she had also appeared in "Miracle in the Rain" and "Bus Stop".
While she had begun appearing on TV in its earliest days, and had worked in many episodes of live anthology series during the 50s, after "The Bad Seed," Heckart had more offers for both the big and small screen, but her heart still belonged to the theatre. "The Dark at the Top of the Stairs" (1957) firmed her place as a Broadway star; yet she squeezed in appearances in such films as "Hot Spell" (1958) and "Heller in Pink Tights" (1960), both for Paramount. In 1965, she created the role of Mrs. Banks in the Broadway production of "Barefoot in the Park," while also appearing in episodes of such TV series as "Gunsmoke" and "The FBI". "Save Me a Place at Forest Lawn" and "The Effect of Gamma Rays on Man-in-the-Moon Marigolds," both for PBS in 1966 won Heckart a host of television awards. She continued to create roles in Broadway comedies and dramas, including Mrs. Baker in "Butterflies Are Free" a 1969 effort which she recreated in the 1972 film version. As the overprotective mother of a blind son, Heckart held her own against Goldie Hawn and Edward Albert and was rewarded with a Best Supporting Actress Oscar. It was two years before Heckart appeared in her next film, "Zandy's Bride" (1974) but in between she starred in plays not only on Broadway, but in Connecticut and New Jersey.
Instant TV recognition for Heckart came in 1975 when she was cast as Mary Richards' Aunt Flo Meredith, a name-dropping, globe-trotting, groundbreaking female reporter on the "Mary Tyler Moore Show". Aunt Flo burst into the WJM newsroom and into the series' audience's hearts, especially when she had an on-again, off-again relationship with Lou Grant (Ed Asner) in subsequent seasonal appearances. Heckart's appearances on the Moore series triggered a spurt in her TV "Q" rating, and she was cast as a regular on "Out of the Blue", an ABC series in and as Eleanor Roosevelt in the NBC miniseries "Backstairs at the White House" (both 1979). Heckart's take on the First Lady was like none other audiences had seen -- uppercrust, to be sure, but urban, vibrant, buoyant, and with a rousing edge of middle-aged sexuality. It was perhaps the first time one could believe Eleanor Roosevelt had actually slept with Franklin, and not just been his saintly "eyes and legs." In 1980, Heckart reprised her role as Aunt Flo, but this time in a fully dramatic way, as a weary reporter on the press bus during a gubernatorial campaign on "Lou Grant" It was one of the first times a sitcom character had been used in a dramatic series on TV. Heckart again tried regular series TV with "Trauma Center" (ABC, 1983) and "Partners in Crime" (NBC, 1984) but with little luck. She was again in support of Mary Tyler Moore on the one-season "Annie McGuire" (CBS, 1988-89). Heckart waited several years before taking another stab at sitcom success as the sharp-tongued mother-in-law on "The Five Mrs. Buchanans" (CBS, 1994). Still, despite additional TV-movie and feature film appearances, and unlike many of her contemporaries who abandon theatre once past middle age for the easier workloads of TV and film, Heckart returned to the grease paint and the live audiences whenever she had a chance, including 1995's "Northeast Local" opposite Mary Elizabeth Mastrantonio and Anthony LaPaglia.