Born Nedinia Hutton, she was the daughter of Wall Street millionaire E.F. Hutton and Post Cereal heiress Marjorie Merriweather Post, an American aristocrat and socialite. After one year of college, she dropped out to enroll at the American Academy of Dramatic Arts. For a decade she worked in stock, on Broadway, and on TV, then made her screen debut in Desk Set (1957). She played cool, aloof women in a number of movies through the mid '60s, then took a decade off from film acting after she married actor Cliff Robertson in 1966. Her TV credits include roles in the series Checkmate and Cannon. She returned to the screen in occasional roles from 1975. She later divorced Robertson. In 1988 she and former actor Ted Hartley, an investment banker, started Pavilion, a film and entertainment development and production company; in 1989 they acquired RKO, forming RKO Pavilion, with Merrill as creative director. She continued working throughout the rest of her life, but focused the majority of her attention to philanthropies. Merrill died in 2017, at age 93.
- Mother Marjorie Merriweather Post was heiress to the Post Cereal company; Merrill's childhood winter home was the Mar-a-Lago estate, which was bought by Donald Trump in 1985.
- Though her family was very wealthy, she supported herself by earning $10 an hour modeling clothes for Vogue magazine.
- Outside of her successful motion picture work, she also appeared in several popular TV shows such as Bonanza, Batman and Mission: Impossible.
- Was an early ambassador for and the face of JDRF, the leading global organization funding type 1 diabetes (T1D) research.
- Created JDRF's annual Rumbough Award for scientific excellence, in recognition of outstanding achievement in diabetes research after the passing of her diabetic son David Rumbough in 1973.