Public services operate increasingly through financialising policy technologies in which governments and other funders ‘invest’ in programmes and interventions that can measure and monetise their social impact. This article investigates this shift towards social investment, focusing on the UK government’s flagship youth programme the National Citizen Service and UK government Treasury guidance, particularly the ‘Green Book’ (HM Treasury, 2018). We argue that policy on social value operates in conjunction with new approaches to impact measurement creating a ‘social investment machine’. The machine operates through innovations in policy alongside ‘evaluation entrepreneurship’ at a programme level to reposition young people as the subjects of investment with imagined futures as economically productive citizens, while their data becomes the currency of investment. This shift towards financialisation in policy also promotes ‘high volume’ services, which in contrast to universal welfare services obscure the structural inequalities that shape young people’s lives.