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).