Current research in public management generally hypothesizes that the involvement of external stakeholders by governments positively affects the performance of policies. Recent research, however, has demonstrated diminishing returns of involvement on performance, as well as different effects of involvement for different types of stakeholder organizations. The present article combines these insights. We distinguish between professional and client-interest stakeholder organizations, and assess the effect of their involvement on policy performance in terms of client outcomes. The hypotheses are tested using a combined longitudinal data set consisting of a representative sample of 69 Dutch local governments and 3,434 clients of the Social Support Act, which aims to increase the independent functioning of individuals with mental or physical impediments. Multilevel analyses show that only the involvement of professional stakeholder organizations is related to policy performance, with negative returns on policy performance at higher levels of involvement.