Standing next to a signpost marking 3,147 miles to New York to the west across the ocean and 874 to John O'Groats in the far north of Scotland, the triple Olympic gold medallist waited torch in hand.
The flame arrived on board a navy search and rescue helicopter after being flown to the nearby Culdrose naval station on a special flight from Athens on Friday evening.
Liuetenant Commander Richard Full stepped off the helicopter with the flame in a small gold lantern from which the torch was then lit.
A crowd of flag-waving spectators, many of whom got up at dawn to be in place on a clear day in Cornwall, cheered and leant out to touch the golden, triangular 'cheesegrater' torch as Ainslie walked slowly past to start the 8,000 mile journey around Britain.
"I'm really very proud for the whole nation," Britain's most successful sailor, with the number 001 on his white London 2012 top, told BBC television.
"It's a fantastic moment. It's such beautiful weather here and so many great people have turned out.
"It was actually pretty emotional to be honest," added the world champion. "So much effort has gone into getting the Olympics here in London, so many people and it means so much."
Ainslie handed over the flame, from meshed aluminium torch to torch, after 300m to surfer Tassy Swallow. She broke into a jog, flanked by a police security team in grey running kit.
Bill Morris, director of ceremonies for London organisers LOCOG, said the 70-day relay would have some very special moments ahead with the torch travelling through England, Scotland, Wales and Northern Ireland.
It will pass through 1,019 cities, town and villages and also make a trip outside the United Kingdom to Dublin in Ireland.
When it finally reaches the capital, in the final week before the opening ceremony at the new Olympic Stadium on July 27, it will even spend a night in the Tower of London in the same vault where the Crown Jewels are kept.
The flame was lit by the sun's rays at ancient Olympia in Greece last week and then formally handed over to Princess Anne at a ceremony in Athens' Panathenaic Stadium on Thursday evening.
The first cauldron, at Culdrose, was lit by former England captain David Beckham.