Promoting population mental health and meeting the burdens of mental illness is a priority public health challenge of the 21st century. But too little attention has been placed on how to design and sustain the scope of strategies and commitments that credibly live up to the full breadth of that challenge. ThriveNYC is an effort by New York City to fill that gap, through a public health approach backed by investment in resources and leadership. ThriveNYC can by example help mobilize a larger community of investigators and policymakers to consider how to meet this challenge, to get to consensus on key elements for effective action and implementation, to reimagine who and what the mental health "system" includes, and, in doing so, to strengthen the social contract that underlies well-being.