The UK has an increasingly hostile immigration policy environment, but is also the scene of substantial pro-migrant civic mobilization. There has been little systematic research on this phenomenon, particularly on what motivates people with relatively secure status to engage with migration as an issue. This paper uses conceptual apparatus from civil society literature to analyse civic mobilization around immigration detention. Drawing on twenty qualitative interviews and an online survey, it shows how immigration detention is comes to be a cause for action: how it was articulated as a cause for concern by research participants in terms of injustice and suffering. It outlines the opportunities for action that exist, to support individuals and campaign to change the system, and reflects on people’s diverse experiences of these engagements. Mobilization is mediated by personal factors: practical constraints limit availability and a range of socio-political positionalities shape people’s engagements with immigration detention. The relevance of the findings to the ‘detention movement’ and to wider migration and civil society debates is explored.