The princely pair competed at a Kent and Curwen-sponsored event at the Hampshire estate of Phantom of the Opera composer Andrew Lloyd-Webber and which benefitted William's African charity Tusk Trust and Harry's Sentebale organization, which helps disadvantaged kids in Lesotho.
And the final score? It was a narrow defeat for the dad-to-be, as in the closing minute Harry won 4 goals to 3.
"We weren't expecting William to be here, it was a lovely surprise. He was on really good form," Tusk's chief executive Charlie Mayhew, one of the dignitaries at the 450-guest event, told PEOPLE.
"He seemed incredibly relaxed, enjoying his polo – just the same as he ever is. He clearly enjoyed the game. It was a tie right up until the last minute when Prince Harry scored."
William, 31, has ended his current series of shifts at the RAF Search and Rescue base in North Wales so he can be by Kate's side when she gives birth in the coming days.
Charles Sainsbury-Plaice / Sentebale and Tusk Trust