Born to Wall Street financier E. F. Hutton and Post cereal heiress Marjorie Merriweather Post, a debutante-age Merrill gave up college after one year and moved to New York to study at the American Academy of Dramatic Arts. She debuted on Broadway in "The Mermaids Singing" (1945) and kept busy for the next decade with acting studies, Broadway and stock theater roles and work in early TV anthology dramas. Already in her thirties, Merrill made her film debut with a sweet performance as part of Katharine Hepburn's reference department staff in "Desk Set" (1957). Several leads soon presented themselves; the biggest hit film of these, "Operation Petticoat" (1959), cast the attractive and self-assured Merrill as one of a bevy of nurses who end up on a broken-down Naval submarine.
By 1960, though, Merrill was alternating supporting roles with occasional leads, and was spending much of her time doing tours and regional work on stage. Film work capitalized on her off screen lifestyle, as in "The Young Savages" (1961), with Merrill as crusading DA Burt Lancaster's socialite spouse, and "Butterfield 8" (1960), as the colorless, long-suffering wife that wealthy Laurence Harvey cheats on with Elizabeth Taylor. Later in the decade Merrill began acting in TV-movies, providing gracious professional turns over the years in "The Sunshine Patriot" (1968), "The Letters" (1973), "The Tenth Month" (1979), and "Fear" (1990). Merrill also racked up over 100 guest credits on various TV series, and tried one herself, "Hot Pursuit" (ABC, 1984). The premise of a couple accused of murder attempting to find the real killer themselves was a neat revamp of "The Fugitive", with Merrill as the victim's rich and vengeful widow, but the show never caught on.
Feature work resumed in the mid-70s, and Merrill gave an excellent performance as part of a well-to-do but wacky marriage party in Robert Altman's underrated satire, "A Wedding" (1978). Subsequent film work has been erratic, ranging from interesting, offbeat items (Altman's "The Player" 1992, the odd mystery "Suture" 1993) to misfires and genre fodder ("Twisted" 1986, "Caddyshack II" 1988). Stage work continued with "Angel Street" (1976) and "On Your Toes" (1983), but Merrill's primary interest became Pavilion, a film and entertainment development and production company she formed with Ted Hartley in 1988. The two married in 1989 and that same year acquired RKO Pictures, renaming their company RKO Pavilion. Merrill is vice chariman of the company.
Merrill's second husband (1966-89) was actor Cliff Robertson, Shame to her Calamity Jan in two guest spots on TV's "Batman" in the 60s.