This paper investigates the effect of ownership on sustainable development and environmental policy in Italian municipally owned corporations (MOCs) dealing with urban waste management, trying to understand if multiple ownership can generate better performance compared to single ownership and if the presence of private partners could be crucial in this. The research question is answered by analyzing 41 MOCs of the largest Italian cities operating in urban waste management. In this specific sector, public administrations have tried to improve their offer in terms of sustainable development, environmental policy, and efficiency. The paper covers the Italian case study, where urban waste production per capita is higher than the European average figure. The waste cycle management service is operated in Italy at local level through totally publicly owned companies (monoadministration or multiadministration), mixed (public–private) companies, or via a full externalization achieved by means of public tenders. Through a multinomial ordered probit panel, we show that MOCs with multiple owners perform better than those having a single owner and perform much better with the presence of a private partner, confirming those academic findings according to which collaborative arrangements can increase efficiency, do better than public sector bureaucracy, and lower the costs of service provision. The results of this paper can be used by academics, practitioners, and policy makers alike. To the best of our knowledge, this is the only study that applies this perspective to the analysis of the current international waste management scenario with relation to MOCs having multiple ownerships.