He was born in Amersham, Buckinghamshire. His mother was Brazilian and his British father travelled the country as a salesman until he became ill and died when Bill was 12. The family moved home many times during Bill's childhood and he lived in Hereford, Glasbury, Colnbrook, Newbury, Erith, Belvedere, Blackheath, Storrington, Westcliff-on-Sea, Wilmington and Worthing. His education was disrupted by the moves and he attended many schools including an independent convent school, a small independent school, followed by Frensham Heights School in Surrey, Dartford Technical College and Southend College.Bill also has a son born in 1966 named Jonathan James Pertwee, he has been in various programs and is a large fan of Clint Eastwood.

His brother James (known as "Jiggy") was an RAF Whitley Bomber pilot who was killed in a crash following a leaflet drop over Dortmund, Germany in June 1941.

Upon leaving school in wartime Bill worked for a company that made parts for Spitfire cannons. He was declared unfit for RAF service himself as he was on medication following a swimming accident, but was a member of the Air Training Corps (UK Air Cadets). He then worked as an accounts clerk at the Stock Exchange and as a salesman for the clothing retailer Burberry in London.

He appeared alongside Kenneth Horne, Kenneth Williams, Hugh Paddick and Betty Marsden in the radio comedy series Beyond Our Ken (1959—64) and Round the Horne (1965—67). He was also a warm-up act for many television shows.

His most prominent role was that of ARP Warden William Hodges in Dad's Army, which he played in both the original television series and the radio remakes, as well as the radio sequel, It Sticks Out Half a Mile, set after the war, in which Hodges takes up a surprising new project.

During his lengthy career, he found time to appear in three Carry On films. These were Carry On Loving, Carry On At Your Convenience and Carry On Girls. However, his performance in At Your Convenience was cut from the final film. Other film appearances include the Lust segment of The Magnificent Seven Deadly Sins.

In 1981 Bill Pertwee played the Demob Centre supervisor in the final episode of It Ain't Half Hot Mum. He later played Police Constable Wilson in another creation of Jimmy Perry and David Croft, You Rang, M'Lord? (1990—93).

Pertwee is president of the Dad's Army Appreciation Society and the author of the book "Dad's Army ... The Making of a Television Legend". A friend of most of the cast of the show, he is also the godfather of one of the sons of Ian Lavender (aka Private Frank Pike). Following the death of his wife, he now lives in Surrey, England. He was awarded an MBE in the Queen's 2007 Birthday Honours list for his services to charity.

In July 2008 he and other surviving members of the Dad's Army cast gathered together at the Imperial War Museum on the 40th anniversary of the show's first broadcast in 1968. On Thursday 31 July 2008 he appeared on This Morning talking about Dad's Army with co-stars Frank Williams and Ian Lavender. On Sunday 3 August 2008 he appeared on BBC 1's Jonathan Ross Salutes Dads Army.

Bill is also Vice President of the "Railway Ramblers". Railway Ramblers