Claude Gagnon presented his first couture collection in Jonquière in 1978 when he was 20 years old. He then studied fashion design at LaSalle College, graduating in 1980. The same year, he worked as an illustrator on Jean-Claude Poitras’ fall-winter collection for Beverini Inc. and opened his own ready-to-wear atelier, Trivia Modes, on Ontario Street West in Montreal. In 1982, he founded the Claude Gagnon Design Inc. label, specialized in the production of high-end ready-to-wear collections. In 1983, after coming in as a finalist twice in the Clairol Fashion Awards, Gagnon was asked by the Quebec Ministère de l’Industrie et du Commerce to represent the new generation of designers on the board of directors of the Centre de promotion de la mode de Montréal.
Around the same time, in an effort to promote Quebec fashion and its designers, he also founded the Association des nouveaux créateurs de mode du Québec (ANCMQ), serving as its president.
In 1985, this rising star of the fashion world designed the dress Céline Dion wore at the Félix awards ceremony, a controversial new look that many critics felt was a little too bold for the young singer, only 17 years old at the time. Claude Gagnon further enhanced his renown by designing the hostess uniforms for Rendez-Vous 87, a special two-game series between a team of NHL All-Stars and the Soviet Union national team. Still in 1987, he took part in a trade mission to France for the Quebec Ministère de l’Industrie et du Commerce, presenting his creations at the 2nd international fashion festival in Paris. Even though his business was expanding, he became a victim of his own success because he was unable to secure the funding needed to fulfil the large number of orders from some 30 boutiques. He had to close his studio in 1987, deciding to focus instead on creating more mass-market ready-to-wear collections for manufacturers. Selected as one of six designers to represent Canada in France, in 1988 he created a special collection for Club Création, a showcase featuring linen fabrics organized by La Maison du Lin. The same year, he worked with the company Pretty Talk Inc., then collaborated with various other Montreal manufacturers throughout the 1990s like Regalia, Ernest, Relance Inc., Conveco Manufacturing and the Zoé label by Michael Phillips Ltd.
In 2008, Claude Gagnon turned his focus to offering custom tailoring and alteration services. His archives are preserved at the McCord Museum.