A rower for CA Nantes of France, Jean Marcel Cosmat captured eight league titles in his home country from 1932 through 1936 and was a member of European Championship-winning teams in the coxed four events of 1934 and 1935. At the 1936 Summer Olympics he, alongside brothers Fernand and Marcel Vandernotte, Marcel Chauvigné and coxswain Noël Vandernotte captured a bronze medal in the same event, falling short of the Swiss and German teams, whom Cosmat believed had been given far more opportunities to prepare for the event. He continued competing for several years, but retired at the onset of World War II. By 2008 he was the oldest Olympic medalist in France and was made an Officer of the Ordre national du Mérite, alongside Noël Vanderotte, who was the only other surviving member of the team. He died several months shy of his 100th birthday, after falling down the stairs in his home in Nantes.