The article contributes to the study of the intersections between institutional and non-institutional politics through an analysis of politicians from activist backgrounds who continue to cooperate with social movements while in office. Previous research on institutional activism and dual militancy has paid inadequate attention to the differences between politicians and civil servants in this respect even though these two groups of actors face differing constraints and opportunities for promoting movement agendas within political institutions as well as for participating in movement activities such as protests. The article maintains that to best understand the relationship between these 'activist politicians' and social movements, it is necessary to consider the micro, meso and macro levels alike. In other words, the overall relationship is affected by the politicians' identities and role properties, the organisational characteristics and dynamics of movements and parties, in addition to structural and cultural factors. The empirical case discussed in the article concerns migrant rights activism in Finland in the late 2010s.