Prince William joined 81,000 people around the United Kingdom to run a mile for BBC's Sport Relief campaign.
The prince was among thousands of runners taking part in the London Mile along the Embankment on Saturday,
London was just one of 144 similar events taking part simultaneously at 1300 BST on Sport Relief Saturday.
Described as "the biggest mile event in history", the race is expected to raise hundreds of thousands of pounds for children in poverty.
Before the run started at London's Embankment, Prince William jokingly said he was in great condition.
"I've been training for about two minutes. I've had a good run round the block," he said.
The prince's appearance was kept secret until just before the race in London started.
"I've run a few miles in my time, not very well. I'm better at being stuck in the swimming pool. It's brilliant, it's an awesome sight what's going on all over the country," the prince said.
Prince William, 22, was expected to run with his brother Prince Harry, 19, but he had to pull out because of an injury.
Lord Coe, the former UK Olympic medal winner and Chair of London's 2012 Olympic Bid, started the race.
The prince, who was also joined by celebrity chef Gordon Ramsay and comics Patrick Kielty and Nick Hancock, took six minutes and ten seconds to finish the race.
At the end he said he thought his brother would have finished the race a lot quicker than him.
Sport Relief was set up by Comic Relief and BBC Sport to help disadvantaged people in the UK and abroad.
Prince William and comedian Patrick Kielty running for Sport Relief
Programming of the event will begin on BBC One at 1900 BST on Saturday.
Their 'Go the extra Mile' campaign slogan was dreamed up in recognition of the 50th anniversary of Roger Bannister running the first sub-four minute mile.
Martyn Smith, senior producer of the Sport Relief campaign at the London Mile, said: "It was fantastic to see Prince William leading off 10,000 people at the Embankment.
"The Sport Relief Mile was the perfect event for all those people, especially families, who have always wanted to enjoy the London Marathon experience but aren't able to run that far.
"The Mile is achievable for all and we are hugely grateful to all the people that helped to organise or take part in the miles all around the UK.
On Friday, the Prime Minister Tony Blair was joined by stars such as Sir Steve Redgrave for a charity mile run at his country residence, Chequers, in Buckinghamshire.
And on Saturday runs around the UK persuaded runners, walkers, hoppers, skippers and jumpers of all ages and abilities to go the extra mile and raise money for Sport Relief.