At Home/Chez Soi was a Canadian research demonstration project that tested the impacts of the Pathways Housing First model on people experiencing serious mental illness and homelessness in 5 cities across the country. In this article, we tell the 10-year story of At Home/Chez Soi, its positive outcomes, and how it contributed to transformative change in public policy from “treatment first” to “housing first” to end homelessness for individuals with severe and persistent mental illness. The lessons learned from this story demonstrate how psychology can influence public policy. In the policy arena, psychologists can play several roles, including conceptualizer or innovator, researcher or evaluator, partnership-maker, clinician or supervisor, policy advisor, knowledge translator, and advocate. All of these roles are important for making policy change. Implications for training in psychology are discussed.