In 2014, New York City launched its Pre-K for All program, which rapidly tripled the number of children in free, full-day prekindergarten. Two years later, the city rolled out ThriveNYC, a citywide mental health initiative with a focus on early childhood. By this time, a team from New York University's medical school had partnered for nearly two decades with the city's Division of Early Childhood Education, during which time they developed, tested, and refined ParentCorps, an intervention aimed at supporting the parents and teachers of prekindergarten children. They were thus well positioned to take on the citywide scale-up of their proven intervention. Nonetheless, the partnership was challenged by the scope of the scale-up and by the need to modify the intervention, which had been developed and tested in schools, for use in the community-based organizations that house many of the city's prekindergarten programs. In this article, Laurie Brotman and colleagues describe how their long-established partnership principles--for example, ParentCorps's commitments to racial equity, centering parents' voices, and continuous learning-- helped guide their actions, their strategy development, and ultimately their plan for scaling ParentCorps locally and nationally.