The practice beat Rafael Moneo, David Chipperield, Stanton Williams, Wright & Wright and Classicist Dimitri Porphyrios in the competition.
The office will work on the first phase of Patel Taylor's masterplan to transform the school, which sits on the bank of the River Thames in Barnes, west London.
Working closely with the school, Nicholas Hare and his team will develop the parameters set out in the masterplan into a series of detailed designs for the new buildings.
While details of the commission remain unclear, a spokesman for the school said Nicholas Hare would 'reorganise the site into something very special and exciting'.
Historically, the 20th-century additions to St Paul's have been seen as an architectural disappointment, particularly as the school has a reputation as educationally among the world's best.
The Patel Taylor masterplan is a bid to resolve this.