William was joined by his wife Kate, who smiled and cheered while watching from the sidelines, wearing a white and sky blue patterned dress reportedly designed by Beulah London.
The prince played in defence, chasing the ball with a sturdy backhand swing and scoring two goals in an intense match.
For his second strike, the father-of-three hit the ball with such force he broke his mallet.
William played in a number four shirt on the winning US Polo Assn team, alongside Aiyawatt “Top” Srivaddhanaprabha, the chairman of Leicester City FC.
With Windsor Castle overlooking the Guards Polo Club, the prince faced off against other polo teams, BP Healthcare and Standing Rock, in the round-robin tournament.
The charity cup is raising funds and awareness for 11 charities supported by William and Kate.