The royal trio raced each other leading off a the first leg of a five-person relay, with joggers training for the London Marathon in April.
William, who enjoys a fierce rivalry with both his younger brother Harry and his wife, joked at the start, "Oh my hamstring," before losing to Prince Harry by a fraction of a second.
William, Kate and Harry came ready to have fun. The Duke and Duchess of Cambridge, who have raced previously in America's Cup yachts and on dragon boats, laced up today with Prince Harry hoping to continue their effort to change the conversation on mental health. William laughed, hugging Kate at the finish line, "You nearly beat me."
Prince Harry, who is no stranger to the track, challenged Usain Bolt in his native Jamaica and showed why he was the favorite to win today.
The training day for runners was organized by Heads Together, the charity set up by William, Kate and Harry, an umbrella organization of eight charities devoted to breaking the stigma surrounding mental health and to encouraging young people to open up and ask for help.
"We want to normalize mental health," William said. "We want to get people talking about it, to make it more normal and to reduce the stigma. What you are doing and having so many of you here today, you've already seen the benefit of joining in, by being part of this marathon -- we want to make it a mental health marathon."
Heads Together is the official charity partner of the 2017 Virgin London Marathon. The three young royals were joined by Olympic Marathon runner and world record holder Paula Radcliffe at the event.
The 150 runners who participated today will be raising funds for one of the eight Heads Together Charity partners.