This paper traces what happens to the official process and objectives of a participatory slum sanitation project, the Nirmal Mumbai Metropolitan Region Abhiyan, when non-governmental organizations (NGOs) encounter informalized municipal administration. We do this by describing the implementation phase from the points of view of the actors involved in two cities within the region, analysing them in terms of contraventions and non-enforcement of official objectives and regulations and then relating it back to discussions about both participatory slum sanitation and informality and municipal administration in India. In this case, elected councillors’ territoriality over slums, plus the coordinated actions among them, municipal officials and local contractors undermined the declared physical and social objectives of these programmes. In fact, NGOs quickly began following the rules and practices of informalized municipal administration. These findings are significant for those working on improving basic urban services because where incumbent actors in municipal governance can coordinate to undermine programmes, or alter them to their relative advantage, then participatory development programmes (as currently organized) lack fitness for purpose. This paper also contributes to debates on municipal governance in India by focusing on how political mediators, in this case ward councillors’ bending or breaking of formal regulations and protocols and the municipal officials’ non-enforcement together, informalized the municipal administration of this project.