The Duke of Cambridge completed a gruelling two-day captaincy test on May 31 – just two days before he travelled to London to take part in the celebrations.
Flight Lieutenant Wales, as William is known in the military, will now command Search and Rescue operations in RAF Sea King helicopters.
A spokesman for him said: "Naturally, he is pleased to have achieved this milestone and looks forward to continuing his work with the life-saving Search and Rescue Force."
During ground and air-based tests, the 29-year-old had to search for people in the sea and put out a fire. In addition he was asked to locate two missing kayakers and a yacht.
William has completed two years of flying experience and study since he joined C Flight, 22 Sqn at Anglesey's RAF Valley in September 2010.
His tour of duty will end next year and he must then decide whether to undertake another or concentrate on his work as a royal.