This paper examines multilateral financial mechanisms (MFMs) that address global environmental public good problems. MFMs provide grants to promote sustainable development, jointly producing (global) public benefits and (domestic) private benefits. I model MFMs and show how to implement efficient outcomes through burden sharing. This is the first paper, to my knowledge, to design an efficient mechanism for contexts with joint production. Comparative static analysis reveals how grant disbursements affect public good provision. Counterintuitively, increasing grant funding to a country can decrease its contributions to environmental quality. Moreover, grant reallocations can have varied welfare consequences: they can be redistributive, Pareto improving, or immiserating. I demonstrate how welfare outcomes, both in terms of efficiency and equity, are shaped by the distribution of grants across countries and by substitute and complement relationships between goods.