Federal, state, and local governments have criminalized mental illness by failing to fund necessary community-based mental health services while incarcerating people for behaviors arising from unmet mental health needs. This Article aims to provide a practical blueprint for a litigation-based decriminalization strategy that can be used by both impact litigation lawyers working towards systemic reform and by public defenders and others challenging arrests, convictions, and incarceration of individual clients. The legal theory draws on existing but largely overlooked U.S. Supreme Court precedent supporting the proposition that criminalizing persons with mental illness contravenes the fundamental values of our criminal justice system. Incorporating this legal theory into both individual criminal defense work and impact litigation has the potential to stem the tide of criminalization of mental illness and catalyze policy change on behalf of one of the most vulnerable populations in our country. If successful, this litigation would dismantle the practice of using jails and prisons as proxy mental health care providers, and drive the creation of community-based services.