The 21-year-old and her American boyfriend Dave Clarke made it across the finish line having run the 26.2 miles tied to 32 other friends as members of 'Team Caterpillar', headed up by Richard Branson's children Holly and Sam.
That in itself was a record breaking feat as the gang became the largest number of runners to finish the race tied together.
Cheering on Beatrice were her proud mum and dad and her younger sister Eugenie, who clapped as the team came in after five hours, 13 minutes and four seconds.
And it seems the super fit royal's example has rubbed off on her family. Speaking to the BBC, Sarah said: "For so long I've been trying to get her fit and now she's showing me up today. I've just decided that if she such a good role model to me then next year I'm going to do it."
Other famous faces amongst the 36,000 people taking part in the annual race included Richard Branson, whose Virgin company was sponsoring the event for the first time.
Dressed as a butterfly, the British billionaire made it round the course in five hours, two minutes and 24 seconds.
Natalie Imbruglia, Elen Rives, Jenni Falconer, Lorraine Kelly and Ben Shepherd also took on the challenge, Jenni missing out on her target time of three hours 30 minutes by just 60 seconds.
The run proved too much for chef Gordon Ramsay who was competing along with his wife Tana.
He was forced to pull out after suffering from cramp in his leg.
Meanwhile, at the very front of the pack were Liliya Shobukhova, who became the first Russian woman to win the marathon with a time of two hours, 22 minutes, while Ethiopia's Tsegaye Kedebe, took the men's title with two hours, five minutes and 19 seconds.