To understand how water, sanitation, and hygiene (WASH) coordination leads to humanitarian response outcomes, we conducted a nine-month mixed-method evaluation in three humanitarian contexts (Cox’s Bazar Bangladesh, Democratic Republic of Congo, and Yemen) varying in terms of humanitarian and WASH response and coordination. We completed 93 key informant interviews, 157 online surveys, and monthly activity reports with National Coordination Platform (NCP) staff and partners. We identified a key set of NCP tools, activities, and products that assisted partners in consistently reaching humanitarian WASH outcomes of “making strategic decisions” and “identifying/reducing gaps in the response”, and partially reaching “defining metrics for Monitoring and Evaluation”, “completing Monitoring and Evaluation and evaluating quality”, and “obtaining funding”. Key contextual factors enabling outcomes were stable staff; cooperation with partners; context-specific tools; and, incorporating the humanitarian-development nexus. One activity, three outcomes (“building capacity”, “strengthening relationships”, and “long-term planning”), and two enabling conditions were added into a pre-existing Theory of Change. This revised Theory of Change is being used to improve NCP strategies, including the Minimum Requirements, and ultimately, benefit affected populations.