Doing that, said Ashwin Vasan, the city’s health commissioner, requires “prevention of the drivers of crisis: isolation, unstable housing, unstable clinical care.”

The plan to tackle severe mental illness is part a broader behavioral health policy the mayor laid out on Thursday that includes treatment for addiction and for the mental health needs of children and families. It also takes aim at racial inequities in health care — Black and Hispanic New Yorkers with severe mental illness are far more likely to be receiving inadequate treatment or no treatment at all.

The plan is in some ways an attempt to directly address a problem that has festered for over half a century, ever since the nation moved away from housing large numbers of mentally ill people in psychiatric institutions without setting up an adequate system of community care.

The city’s blueprint for severe mental illness includes building six new community behavioral health centers, with federal funding, for people with low incomes. It calls for $7 million in city money to expand clubhouses, which are widely regarded as a cost-effective way to help people with severe mental illness stay connected to supports.

The plan also calls for creating 15 more mobile treatment teams that will serve over 800 mentally ill people, including five teams dedicated to the most challenging patients who move often between shelters, jails, streets and hospitals.

Those teams, called Intensive Mobile Treatment teams or I.M.T., include social workers, peer counselors and psychiatric professionals. They have a broad purview to offer clients the help they need wherever and whenever they need it, whether that means helping them find housing, connecting them to benefits, or solving day-to-day problems.

Jody Rudin, president of the Institute for Community Living, a nonprofit that operates six I.M.T. teams under contract with the city, applauded the plan.