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
A letter sent to all 33 local authorities and housing providers in May urged them to consider if panels could be flammable.
The action comes as a commuters' group goes to court to seek a judicial review of the franchise.
Michael Bond, the creator of Paddington Bear has died at the age of 91.