Between 1990 and 2010, a gradual process of institutional change has affected Canada's immigration and integration governance regime. The central characteristic of this process is the emergence of a new legitimate institutional group of actors: Canadian provinces. This change corresponds to a federalization of Canada's immigration and integration governance regime. It is a break from the previous pattern of federal dominance and provincial avoidance. It is not the result of diminished federal intervention in immigration and cannot be explained by exogenous shocks. Current explanations of this evolution focus on federal decisions and have trouble explaining provincial mobilization. Using a mechanistic approach to the analysis of social processes and insights on gradual institutional changes, this article demonstrates that provinces have been the central agents bringing about the federalization of Canada's immigration and integration governance regime between 1990 and 2010. Via a mechanism of province building centred on immigration, provinces have triggered and maintained in movement a decentralizing mechanism. The interactions of these two mechanisms, over time, gave rise to the federalization of immigration and integration in Canada.