Since the 2014–2015 so-called ‘refugee crisis’, civil society organisations have increasingly engaged in practices of political contention in the field of migration in Europe. They have done so at the sub-national, national and even transnational/supranational level, shifting their scale of contention in the context of a process of Europeanisation from below. In this contribution I explain why civil society organisations engage, in different ways and to different extents, in such processes of transnationalisation. I will do so by exploring, in the context of a comparative study, the EU-wide network From the Sea to the City, which gathers together a wide and diverse range of civil society initiatives across the continent, with a view to promoting a more open EU migration policy. Building on participant observation, semi-structured interviews and documentary sources, and using qualitative techniques, I will consider the role played by five types of internal dynamics in the decision made by civil society organisations to engage in this European transnational network. In doing so, my contribution will illustrate which internal factors matter more than others, and why this is the case, in the process of transnationalisation of migrant solidarity activism.