Hewlett spent most of his career playing elderly servants, clergymen, and other old man parts. In science fiction, he was in the first installment of the serial Quatermass and the Pit (1959) and appeared on Doctor Who in two serials: "State of Decay" (1980) and "Trial of a Time Lord" (1986). He performed in several Shakespeare broadcasts between 1964 and 1981, including the 1976 Romeo and Juliet (as Friar John) and served as a TV foil for comedians Tony Hancock, Eric Sykes, and puppet Basil Brush.
Hewlett was cast to type as the Aged P. in the 1991 Disney Channel Great Expectations and appeared in two different Lord Peter Wimsey series, in "The Unpleasantness at the Bellona Club" (1972) and "Have His Carcase" (1987, with Edward Petherbridge). Other TV work included episodes of The Avengers, The Saint, the 1965 Sherlock Holmes series, Edward the King, The Cleopatras, and Blackadder (as the Archbishop of Canterbury and his twin brother in "Archbishop").