Kate, who wore a red puffer jacket and black jeans, had carefully tied back her hair on the start line in preparation for the race.
Just before the race began, William joked “Oh my hamstring” – perhaps daunted by the task ahead of him.
However, despite her best efforts the Duchess, 35, who was the only one of the three to wear jeans to the race, finished last, with her husband coming in second to his brother.
After the competitive race, the royal couple were seen laughing and hugging at the finish line.
William could be heard telling his wife “You nearly beat me!” before joking to his teammates that he had “tunnel vision” during the race.
Each royal ran with the relay with a team of runners including athletes Iwan Thomas and Paula Radcliffe and presenter Sian Williams.
Their Royal Highnesses later spoke to the runners, telling them that they hope that this year’s Virgin Money London Marathon can be made into the ‘mental health marathon’.
The charity day was part of a campaign, headed up the royal trio, who want to change the national conversation on mental health.
HeadsTogether is a partnership between eight charities that provide front-line mental health support.
They believe that this year would see a sea change towards mental health.
Earlier this year, Prime Minister Theresa May announced new funding for mental health provision.
Pippa, Henrietta and Will Creasey, who were at the event today, are running to raise money for CALM.
Henri and Will’s father and Pippa’s husband Dr Terry Creasy committed suicide following an acute struggle with depression.
Pippa said: “Suicide was not a word we thought would be associated with us.
“Nevertheless, four years after tragically losing the head of our very special family, we find ourselves hoping to use the experience to raise awareness of what is the biggest cause of death of men under the age of 45 in the UK.
“As incomprehensible as this statistic sounds – husbands, fathers, brothers and sons are being lost to a silent killer that as a society we find difficult to talk about. It is time to start the conversation.
“Which is why we’re pleased to have the opportunity to run the Virgin London Marathon in support for CALM and Heads Together.
“The strenuous training programme and the twenty-six mile sightseeing extravaganza will undoubtedly be an immense physical and emotional challenge.
“We hope the test will be made significantly easier knowing we are running for two fantastic charities that are raising much needed awareness of men’s mental health.”