Merrill was born in New York City on December 29, 1923, although for many years, her date of birth was given as December 9, 1925. 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. Merrill had two older half-sisters, Adelaide Breevort (Close) Hutton (July 26, 1908 – December 31, 1998) and Eleanor Post (Close) Hutton (December 3, 1909 – November 27, 2006), by her mother’s first marriage to Edward Bennett Close, grandfather of actress Glenn Close.

On advice from her half-sister’s (then) husband, she adopted the stage name Dina Merrill, borrowing from Charles E. Merrill, a famous stockbroker like her father. Merrill made her debut on the stage in the play The Mermaid Singing in 1945.

Merrill was married three times. In 1946, she wed Stanley M. Rumbough Jr., an heir to the Colgate-Palmolive toothpaste fortune and entrepreneur. They had three children, Nedenia Colgate Rumbough, David Post Rumbough, and Stanley Rumbough III before divorcing in 1966. Later that year, she wed future Oscar-winning actor Cliff Robertson, with whom she had a daughter, Heather Robertson (1968–2007). The couple divorced in 1986.

During the late 1950s and 1960s, Merrill was believed to have intentionally been marketed as a replacement for Grace Kelly, and in 1959, she was proclaimed “Hollywood’s new Grace Kelly”.

Merrill appeared in numerous television series in the 1960s, such as playing the villain “Calamity Jan” in two 1968 episodes of Batman alongside then-husband Cliff Robertson. She also made guest appearances on two Bonanza episodes as Susannah Clauson, The Alfred Hitchcock Hour episode “Bonfire” (1962), The Investigators, The Bold Ones, The Love Boat; Quincy, M.E.; 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 Agnus Killer”.

Merrill was a presidential appointee to the board of trustees of the John F. Kennedy Center for the Performing Arts, a trustee of the Eugene O’Neill Theater Center, and a vice president of the New York City Mission Society. In 1980, Merrill joined the board of directors of her father’s E. F. Hutton & Co., continuing on the board of directors and the compensation committee of Lehman Brothers when it acquired Hutton, for over 18 years.

In 1989, she married producer Ted Hartley. Two of Merrill’s four children predeceased her.

Her stage credits include the 1983 Broadway revival of the Rodgers and Hart musical On Your Toes, starring Russian prima ballerina Natalia Makarova. In 1991, she appeared in the rotating cast of the off-Broadway staged reading of Wit & Wisdom.

In 1991, Merrill and her third husband, Ted Hartley, merged their company, Pavilion Communications, with RKO to form RKO Pictures, which owns the intellectual property of the RKO Radio Pictures movie studio.

Merrill attended George Washington University in Washington, DC, for one term, then dropped out and enrolled at the American Academy of Dramatic Arts in New York City. She studied acting at HB Studio under Uta Hagen. She received a lifetime achievement award from the American Academy of Dramatic Arts in April 2005.

On May 22, 2017, Merrill died at her home in East Hampton, New York at age 93. She had been suffering from dementia with Lewy bodies.