[3][4] She was the only child of Post Cereals heiress Marjorie Merriweather Post and her second husband, Wall Street stockbroker Edward Francis Hutton, founder of E. F. Hutton & Co.[5] Merrill had two older half-sisters, Adelaide Brevoort Close (July 26, 1908 – December 31, 1998) and Eleanor Post Hutton (December 3, 1909 – November 27, 2006), by her mother's first marriage to Edward Bennett Close, grandfather of actress Glenn Close.

[12] Merrill's film credits included Desk Set (1957), A Nice Little Bank That Should Be Robbed (1958), Don't Give Up the Ship (1959), Operation Petticoat (1959, with Cary Grant, who had been married to her cousin, Woolworth heiress Barbara Hutton), The Sundowners (1960), Butterfield 8 (1960), The Young Savages (1961), The Courtship of Eddie's Father (1963), I'll Take Sweden (1965), The Greatest (1977), A Wedding (1978), Just Tell Me What You Want (1980), Anna to the Infinite Power (1983), Twisted (1986), Caddyshack II (1988), Fear (1990), True Colors (1991), The Player (1992), Suture (1993), and Shade (2003).

She also appeared in made-for-TV movies, such as Seven in Darkness (1969), The Lonely Profession (1969), Family Flight (1972), and The Tenth Month (1979).

Merrill appeared in numerous television series in the 1960s, such as playing the villain Calamity Jan in two 1968 episodes of Batman with then-husband Cliff Robertson.

; Murder, She Wrote; Roseanne, and The Nanny, as Maxwell Sheffield's disapproving and distant British mother.

In 1971, Merrill appeared as Laura Duff in The Men from Shiloh (rebranded name for the TV Western The Virginian) in the episode titled "The Angus Killer".

Her stage credits include the 1983 Broadway revival of the Rodgers and Hart musical On Your Toes, starring Russian prima ballerina Natalia Makarova.