In the 60s, Daniels' stage career included some flops, including the original 1963 production of "One Flew Over the Cuckoo's Nest" with Kirk Douglas and several musicals, including "On a Clear Day, You Can See Forever" (1965). He had his biggest stage success playing John Adams in "1776" (1969), and sparked some controversy for refusing a Tony Award nomination as Best Featured Actor in a Musical, claiming that Adams was the leading role. Daniels last Broadway appearance to date was as the lawyer Frederick Egerman in the Stephen Sondheim-Hugh Wheeler musical "A Little Night Music" (1973).
Although he had appeared briefly in the film "Family Honeymoon" (1949) and had reprised his stage role in "A Thousand Clowns" (1965), Daniels' big feature break came when he was cast by Mike Nichols as Ben Braddock's (Dustin Hoffman) father in "The Graduate" (1967). While the film gave him Hollywood exposure and led to his repeating his Broadway stage role of John Adams in the 1972 film version of "1776" and playing Brooke Shields' father in "The Blue Lagoon" (1980), Daniels never really achieved film stardom.
TV, however, provided the actor with his best roles. His first series was "Captain Nice" (NBC, 1967), in which he was a mild-mannered chemist who drinks an elixir and becomes a super hero. Although the series was short-lived, it was fondly received in critical circles. After portraying John Quincy Adams in the PBS miniseries "The Adams Chronicles" (1976), Daniels landed the role of Lt. Cdr. Kenneth Kitteridge, husband of talent agent Nancy Walker in "The Nancy Walker Show" (ABC, 1976-77) and went on to play the by-the-book district attorney in "Freebie and the Bean" (CBS, 1980-81). The next year, Daniels' finally found his role. Cast as the egotistical Dr. Mark Craig, a brilliant heart surgeon with the bedside manner of a cold shower in "St. Elsewhere" (NBC, 1982-88), Daniels earned two Emmy Awards and co-starred with his wife (who also won an Emmy). Although the show was never a big ratings success, it did wow the critics and had its devoted admirers. Simultaneously, Daniels provided the voice of KITT, the super car, on "Knight Rider" (NBC, 1982-86). Following the demise of "St. Elsewhere", Daniels freelanced in TV and film projects--including a 1990 TV-movie revival of "Knight Rider"--before hooking on with his next series. He was cast as Mr. George Feeny, the omnipresent principal and nemesis to Ben Savage's Cory Matthews on the ABC sitcom "Boy Meets World" (1993-2000).