Born in Hartford, Connecticut, he attended private Bowdoin College in Brunswick, Maine and began acting in 1944, while still in theUnited States Army Air Forces, in Winged Victory. Before entering films, Merrill's deep cultured voice won him a recurring role asBatman in the Superman radio series. His film career began promisingly, with roles in films like Twelve O'Clock High (1949) and All About Eve (1950), but he rarely moved beyond supporting roles in his many Westerns, war movies, and medical dramas. His television career was extensive, if not consistent. He appeared from 1954-1956 as Jason Tyler on the NBC crime drama Justice, about lawyers of the Legal Aid Society of New York. In that series, he was cast oppostie Dane Clark and then William Prince in the role of Richard Adams.
In 1958, Merrill guest starred with June Lockhart in the roles of Joshua and Emily Newton in the episode "Medicine Man" of NBC'swestern series, Cimarron City.
Merrill had recurring roles in Then Came Bronson with Michael Parks and Young Doctor Kildare, both of which lasted less than a season.
In 1964, he starred as city editor Lou Sheldon, in the short-lived CBS drama about the fictitious New York Globe, The Reporter, with Harry Guardino in the title role as journalistDanny Taylor.
Merrill's first marriage, to Barbara Leeds in 1941, ended in divorce in 1950. He immediately married Bette Davis, his co-star from All About Eve, and adopted her daughter Barbarafrom a previous marriage. He and Davis adopted two more children, but divorced in 1960.
Often politically active, he campaigned in 1958 to elect the Democrat, Edmund Sixtus Muskie, as governor of Maine. Merrill also took part in the Selma to Montgomery marches in 1965 to promote Black voter registration. In response to U.S. President Lyndon B. Johnson's Vietnam War policy, he unsuccessfully sought nomination to the Maine legislature as an anti-war, pro-environmentalist Democratic primary candidate.
Aside from an occasional role as narrator, Merrill had essentially retired from the entertainment business after 1980. Shortly before his death, he authored the autobiography Bette, Rita and the Rest of My Life (1989).
Merrill survived his second wife, Bette Davis, by only five months, dying of lung cancer in Falmouth, Maine on March 5, 1990. He is buried there in the Pine Grove Cemetery.
Merrill's television work spanned from 1953-1980. Most of his appearances were in guest-star roles in episodic and anthology series. Among the programs in which Merrill appeared are: The 20th Century-Fox Hour, Wagon Train, Studio 57, Studio One, Playhouse 90, Alcoa Theatre, Rawhide, Laramie, Sam Benedict, Alfred Hitchcock Presents (the episode "Manacled" original air date 1957), Zane Grey Theater, The Twilight Zone, General Electric Theater, Ben Casey, Combat!, The Outer Limits, Bob Hope Presents the Chrysler Theatre, Voyage to the Bottom of the Sea, The Time Tunnel, Marcus Welby, M.D., Medical Center, Kung Fu, and Cannon.
Merrill also served as narrator of the 1972–73 syndicated TV series The American Adventure.