John was born in Preston, Lancashire. At the age of 12, Inman moved with his parents to Blackpool where his mother ran a boarding house, while his father owned a hairdressing business. As a child, he enjoyed dressmaking.
He was educated at Claridge House in Preston, and then a secondary modern. John always wanted to be an actor, and his parents paid for him to have elocution lessons at the local church hall. At the age of 13 he made his stage debut in the Pavilion on Blackpool's South Pier, in a melodrama entitled Freda. Aged 15, he took a job at the pier, making tea, clearing up, and playing parts in plays.
After leaving school, John worked for two years at Fox's, a gentlemen's outfitters in Blackpool. Aged 17, he moved to London to join Austin Reed in Regent Street. Four years later, he left Austin Reed to become a scenic artist with Kenneth Kendall's touring company at a theatre in Crewe, so that he could earn his Equity Card.
John made his West End debut in the 1960s when he appeared in Ann Veronica at the Cambridge Theatre. He also played in Salad Days at the Windmill Theatre in 1975, and as Lord Fancourt Babberley in Charley's Aunt at the Adelphi Theatre in 1979. He also played in many summer shows, and established himself as a dame in pantomime, appearing regularly as one of the two ugly sisters alongside comedian Barry Howard.
In 1976, he was voted "Funniest Man On Television" by readers of TV Times magazine and was also named BBC TV's "Personality Of The Year." John became popular for his memorably camp role of Mr Humpries in 70s sitcom Are You Being Served?. His catchphrase "I'm free" became part of popular culture. John's character Mr Humphries attracted criticism at the height of the department store-based sitcom's success from some gay rights groups who were upset by what they saw as his portrayal of an over-the-top homosexual.
After the end of Are You Being Served?, John became one of the nation's best known pantomime dames and appeared in over 40 pantomime productions across the United Kingdom. In 2004, Inman made additional television appearances in Doctors and Revolver. He lived in a mews house in Little Venice for 30 years. On December 23, 2005, John entered in a civil partnership at Westminster Register Office with his partner of 35 years, Ron Lynch.
In December 2004, John was forced to cancel an appearance in a pantomime as he was suffering from a "hepatitis A" infection, which he had contracted from contaminated food. Following this, he never worked again and he suffered complications from the infection for the rest of his life. John died in St Mary's Hospital, Paddington, London.
On 23 December 2005, Inman entered in a civil partnership at Westminster Register Office with his partner of 35 years, Ron Lynch.