Kashtan was born in Montréal in 1909. In 1927, at the age of 18, he joined the Young Communist League. Two years later, he moved to southern Ontario to organize for the YCL there and then became the League's general secretary in 1930. In 1936, he helped found the Canadian Youth Congress which, at its peak, had over 400,000 members.
After World War II he served as Toronto organizer of the Labor-Progressive Party, as the Communist Party was known from 1943 to 1959, and served subsequently as industrial director, labour secretary and central organizer.
He was a perennial candidate for the Labor-Progressive Party (as the Communist Party was known from 1943 to 1959) and then the Communist Party of Canada and became general secretary of the party in 1965, despite the opposition of party chairman and longtime leader Tim Buck.
Kashtan never succeeded in winning election to the Canadian House of Commons, and retired in 1988. He was an orthodox, pro-MoscowCommunist and consistently supported the Soviet Union through various shifts in policy at the Kremlin. Kashtan opposed Eurocommunism in the 1970s when many other Communist Parties in the west embraced it.
In 1970, Kashtan spoke out against the Front de libération du Québec (FLQ) group in Quebec, describing it as a terrorist organization and claiming that its methods were not consistent with genuine revolutionary behaviour.