Growing evidence highlights the benefits to youth of involvement in community-based participatory research. Less attention has been paid, however, to the contributions youth can make to helping change health-promoting policy through such work. The authors describe a multi-method case study of a policy-focused community-based participatory research project in the Skid Row area of downtown Los Angeles, California, where a small group of homeless youth worked with adult mentors to develop and conduct a survey of 96 homeless youth and used the findings to help secure health-promoting policy change. They review the partnership’s work at each stage of the policy-making process; its successes in changing policy regarding recreation, juvenile justice, and education; and the challenges encountered, especially with policy enforcement. The authors share lessons learned, including the importance of strong adult mentors and of policy environments conducive to sustainable, health-promoting change for marginalised youth.