By the time she was a teenager, Maguy was already working in the Hotel de Rhuys' dining room, and her brother Gilbert, was learning in the kitchen. The siblings so impressed their father with their skills, he opened a summer disco, La Biscorne, and let Maguy and Gilbert operate it; in the winter, the two headed for ski resorts to run popular clubs and restaurants in the French Alps.

In 1972, while Maguy and Gilbert were still both in their twenties, they moved to Paris and opened a small restaurant called Le Bernardin, after an order of monks devoted to good wine and food. With recipes borrowed from family members and hard to get ingredients arriving by train from Britany—Maguy and Gilbert's parents would collect wild fennel and send by train to Paris— Le Bernardin became one of Paris' hottest restaurants. In 1981, Le Bernardin moved to a larger location and a year later, garnered two Michelin stars.

Expanding on their Parisian success, Gilbert and Maguy in 1986 opened a second Le Bernardin, in New York City. Just three months later, they won a four star review from The New York Times, an unprecedented record. After selling the Parisian restaurant to build on their success in America, the Le Cozes opened Brasserie Le Coze in Coconut Grove, Florida, which has been named one of America's best 25 restaurants by Esquire magazine; in 1994, they opened a second Brasserie Le Coze, in Atlanta, Georgia.

With the untimely and tragic death of Gilbert in the summer of 1994, Maguy began focusing her energies on the New York operations. She sold the Coconut Grove restaurant and worked closely with chef Eric Ripert to uphold Le Bernardin's tradition of excellence and innovation. In 1995, Le Bernardin was re-awarded its four star designation by the New York Times.