Thorne was born on 4 March 1954, at the family home in Anstey, a village located near Leicester to Bill Thorne, a Desford Colliery miner, and his wife Nancy. He had two brothers. Thorne was educated at the Thomas Rawlins School in Quorn, and played multiple sports but excelled the most in snooker. He began playing snooker while holidaying in Eastbourne at 14 years old. He left school at age 15 and became an estimator for a glass factory while practising snooker in Loughborough and then Leicester's snooker halls. Thorne became national under-16 snooker champion in 1970, and won National Under-19 Billiards Championship every year from 1971 to 1973.