The man, in his 50s, had collapsed with chest pains in Felixstowe, Suffolk when medics dashed to the scene, including the air ambulance being flown by the 33-year-old Duke of Cambridge on what would become his first completed rescue mission since starting his new job this week.
A crowd of people who gathered were left gobsmacked when they realised the rescue was by royal appointment, 'The Daily Telegraph' reported.
The patient was treated at the scene and then flown 50 miles to the Norfolk and Norwich University Hospital.
A spokesperson for the East of England Ambulance Service Trust said they had been called to the incident.
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She said: "An ambulance, East Anglian Air Ambulance and a rapid response vehicle, which arrived to scene in four minutes, attended and treated a man in his late 50s who had collapsed with chest pain. He was flown to Norfolk and Norwich University Hospital for further care".
Prince William completed a civilian pilot course in September before taking dedicated 999-response training in order to take up his role with the air ambulance service.
Prince William's shift pattern is expected to be four days on, four days off, but will also take into account his official royal duties.
His first shift began on Monday at Cambridge Airport and his salary will be donated to charity.