The Olympics-themed parade runs from Piccadilly to Trafalgar Square where there is live music.
High-jumper Deirdre Ryan and boxer Kenneth Egan, who have qualified for the Games, are among those joining.
There are 220,488 Irish people living in London according to the last census.
Mayor of London Boris Johnson said: 'I'm delighted to once again be backing London's St Patrick's Day celebrations.
"That they are amongst the biggest outside Ireland reflects the longstanding contribution of many generations of Irish people to London life."
More top news
ITV News, Producer gives her take on the lack of provisions for disabled supporters of the beautiful game.
Londoners could be at greater risk from terror attacks if Government ministers cut police budgets, Sadiq Khan today warned.
A freedom of information request has found of the 53,640 council-owned garages, 41%, were found to be standing empty.