The Square - one of London's most lauded restaurants - will now join the MARC (Marlon Abela Restaurant Corporation) portfolio of restaurants, which includes nine outlets in the capital and on the east coast of the US.
Abela said: "I am delighted to welcome the Square into our family of restaurants. I have always admired its strong commitment to modern fine dining, devotion to quality and use of the finest, locally sourced British ingredients.
"The team's approach to rigorous seasonality, passion for wine and dedication to delivering service excellence is in line with our core values."
Meanwhile, Howard, who owned the Square with business partner Nigel Platts-Martin, said he was thrilled to pass the restaurant to such "a passionate restaurateur".
Platts-Martin also owns the Ledbury, Chez Bruce and La Trompette, added: "Phil and I have had a wonderful 25 years at the Square. I am confident that the Square will continue to thrive under the stewardship of Marlon Abela."
Originally opened in 1991 in St James's, the Square was awarded its first Michelin star in 1994. Two years later the restaurant relocated to Mayfair and in 1998 was awarded two Michelin stars. Howard received the Chef Award at the Cateys in the same year.
The MARC portfolio in London includes the Greenhouse and Umu, both of which hold two Michelin stars; Morton's private members' club; Green's restaurant, which is soon to reopen; O W Loeb Wine Merchants; MARC Fine Wines; MARC Patisserie and Bakery. In the US, it runs A Voce restaurants in New York, Morello Bistro in Connecticut, Bistro du Midi in Boston and the FranÁ§ois Payard bakeries.