Berthier studied at the École Nationale Supérieure des Arts Décoratifs in Paris before working for several industrial design companies, including Mobel and Airborne. In 1967, he founded the design studio ELA (Études et Liaisons avec l'Industrie) with Pauline Deltour and Gérard Mialet.
Over the course of his career, Berthier has designed a wide variety of products, from office accessories to cars. He is best known for his work on the Tykho watch series and the "Le Portable" cell phone, both of which won prestigious design awards.
Berthier also taught industrial design at the École Nationale Supérieure de Création Industrielle in Paris for several years. His contribution to the discipline has been recognized by numerous awards and distinctions, including the Gold Medal of the Académie d'Architecture in 2010 and the Prix du design industriel de la Ville de Paris in 2012.