This working paper addresses the topic of cities' trajectories of participation in Transnational City Networks (TCNs) on migration-related issues to identify factors and mechanisms of mobilisation. We present the results of a qualitative study on Turin (Italy) and Saint-Etienne (France). Both cities started to mobilise internationally in the 1990s on the initiative of entrepreneurial mayors. Yet, throughout the 2000s, they took opposite paths. In Turin, intense participation until 2014 was followed by partial disinvolvement and then, since 2018, by renewed activism. Saint-Etienne started to distance itself in 2008 and has not actively participated since then. We show how in the case of Turin, internationalisation has been driven by the engagement of network professionals' personal relations in boundary-spanning work, i.e. in establishing connections between public and non-public actors and between the local and international spheres. Such dynamics are absent in the case of Saint-Etienne, where mobilisation in TCNs on migration has always been mayors-centred.