This article contributes new insights to research on citizenship and young citizen subject formation in the context of the posthuman condition. Bringing a feminist materialist sensibility to bear, we explore citizenship as "materially" mobilised and produced. Considering the constitutive role that embodied and affective phenomena play in this production, we attend particularly to acts of citizenship. We show by way of vignettes how human subjects and material and natural objects 'intra-act' to produce civic capacities and bring citizen subjectivity into effect. The forces by which these capacities are produced come into view inviting challenge to normative, human-centred framings of (youth) citizenship. The forces in question are various--affective, corporeal, temporal, spatial, spectral--but it is affect that provides the main impetus to action. Supplementing the more conventional frame of citizenship as belonging, we propose a framework of citizen "becoming" as a generative way to think and do citizenship in the posthuman present. The argument is made that analytic frames that attune to citizenship as an affective movement of becoming best address current conditions for producing citizen subjects and usefully extend individualised models of citizenship that have long influenced education.