John Nettles was born in St Austell, Cornwall in 1943. He was adopted at birth by a carpenter Eric Nettles and his wife Elsie and he attended the local grammar school in St Austell.
When he was seven years old he learned that his birth mother was a Catholic Irish nurse who had been working in England during World War II. She was placed into a mental institution after illegitimately giving birth and died of tuberculosis aged 28. Nettles has never discovered the identity of his father, but did discover he has a brother and two sisters.
In 1962, Nettles won a scholarship and studied history and philosophy at Southampton University. It was at Southampton that he first performed and, having found he liked acting, he was given several theatre parts. In 1969-70 he was in repertory at the Northcott Theatre, Exeter and in the latter year had his first screen role in the film One More Time.
The following year he played Dr. Ian Mackenzie in the period drama A Family at War, a role he continued until 1972. Following that he had small parts in many TV programmes including The Liver Birds, Enemy at the Door and Robin of Sherwood. He married his first wife, Joyce, in 1966. They had a daughter Emma (born 1970) and divorced in 1979.
In 1981, John Nettles won the role that would make him a household name, that of Jim Bergerac in the Jersey-set crime drama Bergerac. This would run for 87 episodes until 1991. Following the end of Bergerac Nettles did five seasons with the Royal Shakespeare Company, appearing in The Winter's Tale, The Merry Wives of Windsor, Julius Caesar, Richard III and The Devil is an Ass.
In 1992, he appeared in an episode of Boon and in 1993, Nettles appeared as Jim Bergerac in the spoof police comedy The Detectives. In July 1995 he married for a second time, to Cathryn Sealey in Evesham, Worcestershire. He had met Sealey while doing a pantomime.
In 1995, John Nettles was approached by Brian True-May to play Tom Barnaby in a new murder mystery series he was to produce called Midsomer Murders. This was to be the second big role of his career, again playing a policeman. Midsomer Murders has made him not only a household name in the UK, but also across the world, and Nettles continues in the role as of 2008.
In 2003, he played Barnaby in the Boxing Day episode of French & Saunders. Nettles narrated the BBC documentary Airport from 1996 to 2005 and appeared in an episode of Heartbeat.
He has also written three books, called Bergerac's Jersey, John Nettles' Jersey and Nudity in a Public Place. In 2007, he appeared in the BBC Radio 4 comedy series Will Smith Presents the Tao of Bergerac alongside comedian Will Smith about an obsessive fan of the series.