The pair, who opened Kitchen W8 together in Kensington in 2010, will open Sonny's Kitchen in Barnes, on the site of Mascarenhas's restaurant Sonny's in May.
As with Kitchen W8, Howard will have control over the kitchen and menus while Mascarenhas will concentrate on the business and operational side of the venue.
Mascarenhas, who has owned the Sonny's site for the past 26 years, said the partnership at Kitchen W8 had worked well, so it seemed like a natural step to get Howard to work with her at Sonny's where the chef has also been a regular for the past 10 years.
“We began our professional journey together with Kitchen W8 and quickly realised that we complement each other’s skills," she said. "Phil is an undisputed talent in the kitchen, and, after nearly 30 years in the business, I bring strong business acumen to the table.
“Transforming Sonny’s into Sonny’s Kitchen is a timely joint endeavour – and we are thrilled to be working together on this second project, so close to both our homes."
Sonny's Kitchen, which will be re-designed by Design LSM, will continue to be a neighbourhood-style restaurant open for lunch, dinner and weekend brunches when it re-opens mid-May.
Menus intend to be informal and family-friendly and it will be the first time that Howard, who has been head chef and partner of The Square in Mayfair for the last 21 years, has moved into a more relaxed form of cooking.
The pair will continue to run Kitchen W8, which won its first Michelin star in 2011, while Howard will remain partner and head chef of The Square in Mayfair and partner of The Ledbury in Notting Hill. Mascarenhas also owns Cantinetta, an Italian restaurant in Putney, London.