The appointment will surprise many observers, who had tipped Hopkins Architects to win the high-profile job - a multi-million pound revamp of the Grade-I listed Georgian masterpiece, which will initially focus on creating a new café.
English Heritage (EH) unexpectedly decided to plump for the Stirling-shortlisted practice, even though Hopkins had already been asked to draw up concept designs before the project was put out to tender.
The proposed overhaul of the historic house, considered by some to be the home of landscape design, has not been universally welcomed.
Both the Georgian Group and a handful of locals have raised concerns about the plans, which they believe will spoil the historic setting of the building.
However, EH has received the backing of the Heritage Lottery Fund, which promised nearly £8 million of Stage One cash towards the first phase of the project earlier this year ( Contentious Chiswick House plans win lottery cash).
Full details about the extent of Caruso St John's work have yet to be decided, but it is understood there are eventual plans to build a new facilities building alongside the existing house.
A spokesman for EH said he was 'delighted' by the appointment, which he claimed was a 'quite unanimous' decision from the project team.