A deep-voiced performer, Kilgarriff began his career on stage in pantomime (in imposing roles such as giants and ogres, Aladdin's genie, and Little John), and music hall (often as Mr. Chairman, the master of ceremonies role). His television work dates to 1959, including the regular role of King Edward IV on The Golden Spur and appearances on the comedies Taxi! and Whack-O. He went on to appear several times in heavily costumed roles on Doctor Who, notably as the Cyber Controller in "Tomb of the Cybermen" and "Attack of the Cybermen" and the title character in "Robot." Other on-camera credits include the 1967 film of Camelot and an episode of UFO.
Kilgarriff has been busiest off-screen, heard in hundreds of BBC radio broadcasts, including Shakespeare, The Archers, Cyrano de Bergerac, arious Sherlock Holmes installments, War and Peace, and adaptations of Isaac Asimov and P. G. Wodehouse. He adapted The Hobbit for a 1968 radio serial and has written several books (mainly about music hall or comedy). Other voice work includes Obelix in the English dub of The Twelve Tasks of Asterix, a Death Eater in Harry Potter and the Goblet of Fire, the cartoon seriesOscar's Orchestra, and the video game Dark Souls II.