Daniels began his career as a member of the singing Daniels family in Brooklyn. He made his television debut as part of a variety act (along with other members of his family) in 1943, on NBC, then a single station in New York. He made his Broadway debut in 1943 in Life With Father, and remained a busy Broadway actor for decades afterwards. His Broadway credits include roles in 1776, A Thousand Clowns, On a Clear Day You Can See Forever, and A Little Night Music. He received an Obie Award for The Zoo Story (1960). Daniels’s motion picture debut was as a school principal in the 1963 anti-war drama film Ladybug Ladybug. In 1965, he reprised his Broadway role as a child welfare worker in the screen version of A Thousand Clowns. In 1967 he appeared in The Graduate as the father of Dustin Hoffman’s character. In 1969, Daniels starred as John Adams in the Broadway musical 1776; he also appeared in the film version in 1972. Two years later, he co-starred in Richard Donner’s telefilm Sarah T. – Portrait of a Teenage Alcoholic. In 1979 he again reprised his role as the outspoken John Adams in the film Rebels, again about the American revolution, without seeming to ever break character. He is known Arguably as the quintessential John Adams. Daniels’s first network television appearance came in 1952 when he portrayed the young John Quincy Adams, eldest son of John and Abigail Adams in the Hallmark Hall of Fame drama A Woman for the Ages. In 1976, he reprised the role as the middle-aged and elder John Quincy Adams in the acclaimed PBS miniseries The Adams Chronicles. He starred in the short-lived series Captain Nice as police chemist Carter Nash. He appeared as acid-tongued Dr. Mark Craig in St. Elsewhere from 1982 to 1988, for which he won two Emmy awards. Concurrently, he provided the voice of KITT in Knight Rider from 1982 to 1986. Daniels said in 1982, “My duties on Knight Rider are very simple. I do it in about an hour and a half. I’ve never met the cast. I haven’t even met the producer.” He reprised the voice-only role of KITT in 1991 for the television movie Knight Rider 2000, and again in the theatrical comedy movie The Benchwarmers. He performed the role in AT&T and GE commercials about talking machines, and twice in The Simpsons as well as at the Comedy Central Roast of his co-star David Hasselhoff. He reprised the role of KITT in the 2015 Lego-themed action-adventure video game Lego Dimensions. Daniels portrayed strict but loving educator George Feeny at John Adams High School in Boy Meets World from 1993 to 2000. In addition to the previously mentioned 1967 superhero sitcom Captain Nice, he was a regular on the 1970s TV series Freebie and the Bean and The Nancy Walker Show. A familiar character actor, he has appeared as a guest star on numerous TV comedies and dramas, including Soap, The Rockford Files, Quincy, M.E., Kolchak: The Night Stalker, and many others. In 2012, Daniels appeared in the ninth season of Grey’s Anatomy as Dr. Craig Thomas, an unlikely mentor to the character of Dr. Cristina Yang played by actress Sandra Oh. His character, Dr. Thomas, died in the operating room while performing a procedure to repair a heart defect midway through the season, which forced Yang to move back to Seattle. In 2014, Daniels reprised his role as Mr. Feeny in the pilot episode of the Boy Meets World spinoff, Girl Meets World. He cameoed in the final scene, praising the adult Cory Matthews for his parenting. He made additional appearances in the second and third seasons.