It is a momentous occasion for the new royal.
Accompanied by her husband, the Duchess of Cambridge royal was by the Queen's side as she received well wishes from hundreds of thousands of spectators lining the banks of the Thames.
And the stunning royal certainly dressed for the occasion.
Regal in red, the 30-year-old chose a long-sleeved Alexander McQueen dress that falls in pretty pleats from her slim hips.
She topped off the look with a Sylvia Fletcher hat in the same vivid scarlet hue.
Prince William, meanwhile is resplendent in his RAF uniform.
The pair were on the royal barge during the water extravaganza, which saw a flotilla of 1,000 ships, boats, yachts and other vessels sail the river.