It follows the departure of Amandine Chaignot, who stepped down from the post in January after three years.
The move marks a return to the UK for Cahill, who spent ten years working for the Mandarin Oriental group in Asia, including a stint as executive chef in Taipei.
He joins Rosewood from DUKES Hotel in Dubai, where he led a team of 65 across nine food and beverage outlets.
In his new position the chef will oversee Rosewood’s The Mirror Room restaurant and Scarfes Bar.
Calum Franklin remains executive chef at Holborn Dining Room and the Pie Room, which are also situated in the hotel.
“Rosewood London has such a vibrant and dynamic kitchen team, and it will be a pleasure to work alongside them," says Cahill.
The Holborn site opened in 2013 after an £85m revamp of the former Chancery Court Hotel by its US parent company Rosewood Hotels & Resorts.
The group is set to launch its second London hotel on the site of the former US Embassy in Grosvenor Square.