As a stage actor, Squire performed at Stratford-upon-Avon and at the Old Vic, and notably replaced his fellow-countryman Richard Burton as King Arthur in Camelot at the Majestic Theatre on Broadway.

[1] Squire had many roles in television and movies over his career, including Thomas More in the 1969 film version of Maxwell Anderson's play Anne of the Thousand Days; Sir Daniel Brackley in the 1972 television adaptation of Robert Louis Stevenson's The Black Arrow; the voice of Gandalf in the 1978 animated version of The Lord of the Rings; and the Shadow in the 1979 Doctor Who serial The Armageddon Factor.

[2][3] According to the website Television Heaven, Squire's best-known role was the spy chief code-named Hunter in the British series Callan.

[5] This was in keeping with his long career as a Shakespearean actor, which included roles in the classic 1960s TV series, An Age of Kings.

[7][8] In the late 1960s Squire narrated a series of radio advertisements for Findus Foods under the pseudonym Frobisher Collingwood.