Born in Saint-Mont, France, Lamothe immigrated to Canada in 1953 and immediately got a job as a lumberjack in the Abitibi region of northern Quebec. In 1954 he began studying economics at the
Université de Montréal. During his time as a student he became interested in cinema and began writing for several film publications. After graduating in 1958, he was immediately hired as a writer for
Radio-Canada. In 1961 he was hired by the
NFB and wrote Dimanche d'Amérique, his first screenplay, which became the first film by
Gilles Carle.[3] In 1962 he directed his first film, a short documentary entitled Bûcherons de la Manouane. He directed his first feature-length fiction work in 1965, Poussière sur la ville; the film was not successful and Lamothe returned to documentary films.[4] Lamothe has only made a handful of fiction films during his career, and he is best known and most respected for his documentaries. In his films, Lamothe most frequently explored social and economic themes, as well as activism for issues he passionately supported, especially with regard to Aboriginal people.