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 Roman 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 the University of Southampton. At Southampton he first performed and, having found he liked acting, he was given several amateur 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, Dickens of London, Robin of Sherwood and an episode of Enemy at the Door called Officers of the Law, first broadcast in March 1978. The latter was set in Guernsey during the World War 2 German occupation and Nettles played a police detective ordered to work for the Germans and anguished over the conflict between his duty and collaborating with the enemy.
In 1981, John Nettles won the role that made him a household name, that of Jim Bergerac in the Jersey-set crime drama Bergerac. This ran for 87 episodes until 1991. Following the end of Bergerac Nettles did five seasons with the Royal Shakespeare Company, appearing in A 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 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 major role of his career, again playing a policeman. Midsomer Murders has made him a household name not only in the UK, but also across the world. In 2003, he played Barnaby in the Boxing Day episode of French & Saunders. Nettles narrated the BBC documentaries Submarine and Airport from 1996 to 2005 and appeared in an episode of Heartbeat. 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.
In February 2009, it was announced that John Nettles would be leaving Midsomer Murders after two further series are made. His final appearance on-screen was on 2 February 2011, by which time he had appeared in 81 episodes.[