A stage performer as a child, Daniels made his film debut in 1948 with the comedy Family Honeymoon. From there he returned to the theater, but renewed his film career in the early '60s, playing supporting roles in such notable films as Frank Perry's Ladybug, Ladybug, Fred Coe's A Thousand Clowns, Theodore J. Flicker's The President's Analyst, Mike Nichols' The Graduate, and Stanley Donen's Two for the Road. In the '70s Daniels starred as John Adams in the musical 1776; he also appeared in thrillers such as Alan J. Pakula's The Parallax View and John Frankenheimer's Black Sunday, and comedies such as Carl Reiner's Oh, God! and The One and Only. Daniels starred in the television drama series St. Elsewhere from 1982-88, for which he won two Emmy awards. Other notable films include Warren Beatty's Reds and Blake Edwards' Blind Date.