The petite, light-haired performer began on stage in a 1958 summer stock production of "Roberta" at the North Shore Music Theatre in Beverly, MA, then made her Broadway debut the following year as Thelma in the landmark stage musical "Gypsy". She made her London debut in the title role of "Pocahontas" (1963). Throughout much of the 1960s, Gillette racked up numerous stage credits, in either original NYC productions or revivals throughout the US and Canada. She was cast as Sarah Brown in "Guys and Dolls" both in Toronto (1962) and NYC (1965). Gillette was featured in Irving Berlin's "Mr. President" (1962) and the notorious "Kelly" (1965), created the role of Susan Hollander in "Don't Drink the Water" on Broadway (1966), earned praise as Sally Bowles in "Cabaret" (1968) and co-starred with Frank Gorshin in the short-lived "Jimmy" (1969).

In the late 60s, her Broadway status earned her a place on the numerous panel shows which were televised from New York, including "What's My Line" and "I've Got A Secret". Gillette also appeared on "The Ed Sullivan Show", "The Tonight Show", and Garry's Moore's program. Her stage appearances became more sporadic in the 70s and 80s, although she did return to Broadway in 1984 as Blanche in "Brighton Beach Memoirs". In lieu of theater, she moved towards sitcoms. In 1972, she was Ted Bessell's lady love in "Me and the Chimp" (CBS) and in 1973 was the conservative Alice in the sitcom based on the hit movie about partner swapping, "Bob & Carol & Ted & Alice" (ABC). She also played the wife and mother in Norman Lear's experimental 1979 sitcom "The Baxters", which sought to be interactive before anyone knew the meaning of the word. Gillette also has dramatic credits including the role of Dr. Emily Hanover on NBC's "Quincy, M.E." The character went on to marry the coroner (played by Jack Klugman, her "Gypsy" co-star). When the show ended its run, Gillette returned to New York to appear as a regular on "Search for Tomorrow" during that soap opera's waning days, then returned to Hollywood in 1988 as a regular on the short-lived "Almost Grown", in the first of many subsequent mother roles. Gillette also appeared in a few TV-movies, beginning with "It Happened at Lakewood Manor/Ants" (ABC, 1977). She was the wife whom Bob Newhart leaves behind when he starts to jog daily in "Marathon" (CBS, 1980).

Gillette was slower to make it into feature films, with "Moonstruck" her first significant role. Since then, she has played a drunken woman in "Bum Rap" (1988), the Harrisburg Mayor's Wife fawning all over Tim Robbins' candidate "Bob Roberts" (1992) and two more mother roles in "Larger Than Life" and Edward Burns' "She's the One" (both 1996).