The 'learning city' contains a range of non-formal learning economies. In recent years researchers have focused on, what has been termed, the non-formal arts learning sector, to document best practices, the emergence of new literacies and/or cultural practices, and to highlight interventions that support otherwise marginalised and underserved communities. Yet, for all of this attention, the non-formal learning sector has remained an opaque object, defined by hazy boundaries, diverse programme structures, and a presence in cities that is difficult to grasp. In this paper we develop an account of the non-formal arts learning sector for socially disadvantaged youth by treating it as a 'socio-technical assemblage' of the learning city. We draw on data from the "Youthsites" research project and examine the history, priorities, and tensions in the sector between 1995 and 2015, a period when the youth arts sector has become a significant feature of urban space. We trace the emergence of the sector in three global cities, analyse a series of paradoxes linked to income and property, the labelling of youth, and organisation aims, and show how these paradoxes shape the sector's broader relationship with the state, labour and consumer markets, and related institutions that allocate support for young people.