Prince George Alexander Louis of Cambridge, the first born child to Prince William and his wife Kate Middleton, has been given the honour in British daily The London Evening Standard's Power 1,000 list.
Prince George, third in line to the British throne, was chosen because he has become London's biggest global ambassador, the daily said as it released a teaser of the full list, issuing only the top 20 names.
Mayor Johnson, who had topped the list of a year ago, came in at second. New Bank of England Governor Mark Carney was ranked third because of his power over the economic recovery, above Cameron in fourth and Chancellor George Osborne in fifth.
A panel of Evening Standard critics and specialist reporters picked the Power 1000: London's Most Influential in association with Battersea Power Station.
Editor Sarah Sands said: "London is a magnet for the rest of the world and our newest power resident, Prince George, is a timely symbol. He is our greatest tourist attraction, along with his great grandmother, which is why he has been chosen this year as the first among Londoners."
Five members of the royal family made the top 20, including Kate, Princess William and Harry and the Queen.
Tennis player Andy Murray figures in 8th position while deputy Prime Minister Nick Clegg is 12th in the list. The Queen is listed as 16th.