Chamblain first emerged as a notorious figure as a sergeant within the transitional military junta running Haiti following the collapse of Jean-Claude Duvalier's dictatorship in 1986. In 1987, Chamblain allegedly headed government death squads that interrupted a planned election that would have marked the transition to civilian rule. In all, 34 voters were killed, and the election was cancelled.
Civilian elections did take place in 1990, in which Jean-Bertrand Aristide was elected, but a military coup in which Chamblain was involved overthrew Aristide in 1991. Immediately following the coup, Chamblain's reputation for brutality grew further as he is reported to have been responsible for thousands of murders of Aristide followers. 
Chamblain formed a paramilitary organization, the Front for the Advancement and Progress of Haiti (FRAPH), in 1993 as tensions grew between supporters of Aristide's reinstatement and supporters of the military government. With the end of the military regime and restoration of Aristide (following U.S. intervention) in 1994, Chamblain went into exile in the Dominican Republic. He was convicted in absentia for his involvement in the murder of Antoine Izméry, a well-known pro-democracy activist.
In February 2004, Chamblain returned from exile to take part in a new rebellion against Aristide. Shortly after his return, he captured the central city of Hinche from the Haitian police with a force of 50 men.
Following Chamblain's return and the collapse of Aristide's government in 2004, Amnesty International called for UN peacekeepers to arrest Chamblain for his alleged participation in various war crimes in 1987, 1991, and 1993-1994.