The 28-year-old, who is second-in-line to the British throne, was among the crew of a Sea King helicopter called to a rig in Morecambe Bay, off the coast of northwestern England, on Saturday.
William's office said the crew flew through "squally winds" and mild turbulence to collect the rig worker, who had a suspected heart attack, and transport him to a waiting ambulance.
"Prince William is pleased finally to be able to contribute to the lifesaving work of the search-and-rescue force. He is proud, after two years of intense training, to be able to serve in one of Britain's foremost emergency services," his office said in a statement.
It said the royal's Sea King helicopter, call sign "Rescue 122", was scrambled by the Liverpool coast guard office, and took about half an hour to fly to the rig from the crew's base at RAF Valley, in Anglesey, north Wales, on the Irish Sea coast.
William, known as Flight Lt. Wales in his new job, joined the crew last month after completing his training.
Gas distributor Centrica Group PLC, which owns the rig, said that the sick man was a contractor. The company said he was taken to Blackpool Victoria Hospital, in the northern England coastal town Blackpool, and was undergoing treatment.
Though his younger brother Harry served in Afghanistan in 2008 as a battlefield air controller - until his tour of duty was cut short following a media leak, William is considered unlikely to be allowed to serve in a combat zone.
Last year, he said he remained "hopeful there's a chance" he might serve in Afghanistan. Military chiefs, however, believe William would be put in too much danger.
William is scheduled to spend three years as a search-and-rescue pilot for the RAF, and is likely to be eventually promoted to the rank of captain - meaning he would have overall control of his helicopter crew.
His unit routinely flies missions to Northern Ireland, where ministers and security chiefs have warned of a growing threat from dissident Irish Republic Army terrorism.