[1] His father was an organist at St Stephen's Church, Sydenham Hill,[2] where Frank learned to play the organ for a short while.

After two years working at the insurance company, he was invited to become a day student at the acting school and persuaded his father to finance his studies.

[3] During the Second World War, Thornton was evacuated along with the drama school, and his first job was touring with four plays in Ireland, beginning in County Tipperary.

He appeared in the farce The Party Spirit in the West End alongside Robertson Hare and Ralph Lynn.

After working on stage and in a few films during the 1950s, he became a familiar face on British television, specialising in comedy but initially starred in the TV series William Tell as Heinburgher in episode 23, "The Surgeon".

He was a regular on It's a Square World, and appeared in British sitcoms such as Hancock ("The Blood Donor", 1961), Steptoe and Son, Sykes, The Goodies and Love Thy Neighbour.

He worked with Dick Emery, Benny Hill, Frankie Howerd, Harry Worth, Reg Varney and Spike Milligan in their comedy shows and appeared in five episodes of Steptoe and Son during its first run from 1962 to 1965, and appeared in the film Steptoe and Son Ride Again (1973) and the 1973 television Christmas special, 'The Party'.

He continued to appear in films, mostly comedies, during the 1960s and 1970s, including Carry On Screaming!, The Early Bird, The Big Job, The Bed Sitting Room, Up the Chastity Belt, Some Will, Some Won't, A Funny Thing Happened on the Way to the Forum and No Sex Please, We're British, as well as television sitcom spin-offs.

In 1984 he starred as Sir John Treymane in the hit London musical Me and My Girl, earning rave reviews and an Olivier Award nomination.