Lanctôt joined the Front de Libération du Quebec (FLQ) terrorist group in 1963 at the age of 17 and was involved in several violent demonstrations in Quebec during the 1960s along with his sister, Louise Lanctôt.
In 1963, the FLQ was organized and trained in terrorism by Georges Schoeters, an itinerant Belgian revolutionary, whose hero was Che Guevara. At least two of the FLQ members had also received guerrilla training in selective assassination from Palestinian commandos.
From 1963 to 1970, the FLQ committed over 200 violent crimes, including robberies of dynamite, bombings, bank hold-ups, and at least three violent deaths by FLQ bombs and two murders by gunfire.
In 1966 a secret eight-page document entitled "Revolutionary Srategy and the Role of the Avant-Garde" was prepared by the FLQ outlining its long-term strategy of successive waves of robberies, violence, bombings and kidnappings, culminating in insurrection and revolution.
In 1968, Lanctôt, a member of the FLQ's Liberation cell, met Paul Rose, leader of the Chénier cell.
As a member of the Liberation Cell of the FLQ he was partners with:
On December 3, 1970, Lanctôt and the four other known FLQ members who had kidnapped James Cross negotiated his release in exchange for a flight to Cuba. Later, he secretly left Cuba and went to live in Paris, France.
Jacques Lanctôt returned from exile in Paris on January 11, 1979. He was arraigned in Court and released on bail pending his trial. He was also charged with the February 1970 conspiracy to kidnap Moshe Golem, the Israeli trade commissioner to Canada.
In total, Jacques Lanctôt served one year in jail for his crimes.