Hewitt was born in Derry, Northern Ireland, and grew up in Kent and Devon. He was educated at Millfield, a public school, in Street, Somerset.
Having attended the Royal Military Academy Sandhurst, Hewitt was commissioned into the Life Guards, British Army, on 8 April 1978 as a second lieutenant. He was promoted to lieutenant on 8 April 1980. He transferred from a short service commission to a special regular commission on 1 October 1981. He was promoted to captain on 8 October 1984. On 21 October 1985, he transferred from a special regular commission to a regular commission. In 1991, he served as a Challenger tank commander in the Gulf War. He was mentioned in despatches 'in recognition of service during the operation in the Gulf' in June 1991. He failed the exam for promotion to major three times. The rank of Major was conferred on Hewitt after he had retired.
On 1 March 1994, he was retired from the British Army after 17 years' military service.
Hewitt opened a golf driving range in 1994. He opened The Polo House, a bar in the fashionable Golden Mile of Marbella, Spain, in 2009. It closed in 2013.
In 2006, Hewitt appeared as a contestant in The X Factor: Battle of the Stars, the celebrity version of The X Factor, in a duo with Rebecca Loos. He also appeared in the first episode of Top Gear UK's Series 8 in May of the same year to participate with other celebrities in setting power lap times around the Top Gear Test Track in the show's newest "reasonably priced car": when the presenters seemingly did not recognise him upon his arrival, they were apparently too embarrassed to admit as much, and rather than ask him his name, they listed his lap time as "Well Spoken Man".
In 1994, six months after his retirement from the military, Anna Pasternak published the book Princess in Love. Hewitt was a major source, and it alleged that Hewitt had had a five-year affair with Princess Diana, from 1986 to 1991. Diana confirmed the affair in her 1995 Panorama interview.
In 1996, the film Princess in Love by David Greene was released, based on the book. Julie Cox and Christopher Villiers were the actress and the actor who played Diana and Hewitt respectively.
Hewitt had considered committing suicide after his affair with Diana ended. He was preparing for a trip to France, and he wanted to shoot himself. He said, "I got in my car and loaded a few things up to get on the ferry to go to France – to shoot myself .... And then my mother insisted on coming with me. And, if she hadn't, I would have probably shot myself. So I owe her my life really".
Persistent suggestions have been made in the media that Hewitt, and not Charles, is the biological father of Prince Harry. Hewitt stated to the press in 2002 that Harry had already been born by the time Hewitt and Diana began their affair, a statement also made by Diana's police bodyguard. He reiterated this in an interview in 2017.
In 2003, Hewitt tried to sell his 64 personal letters from Diana for £10 million. The act of selling the letters was considered to be a betrayal of trust, and Sarah, Duchess of York, condemned his action. She was reported to have said, "Betrayal, I think is the most horrible, horrible, disloyal thing you can do to anyone".
In July 2004, Hewitt was arrested outside a restaurant in Fulham with Alison Bell, a CNN journalist, for the possession of cocaine. A drunken Hewitt had 0.36 grams (0.013 oz) of cocaine in his pocket. He was given a warning, and Bell was released without charge. As a result, he was refused reinstatement of his firearms licences because of his "intemperate habits" after police found a disassembled 16-bore shotgun on his living room floor.
On 14 May 2017, it was reported that Hewitt had suffered from both a heart attack and stroke, leaving him fighting for his life in hospital.