Mosher has produced thirty-seven books, either collections of his own works or books that he has illustrated. His latest collection, published in the fall of 2004, is entitled OH, OH!.
Mosher est né à Ottawa et a fréquenté quatorze écoles différentes à Montréal, Toronto et Québec et, en 1967, il a obtenu un diplôme de l'École des Beaux-Arts de Québec.