In an effort to address one of New York City’s greatest challenges as the city recovers from the pandemic, Mayor Eric Adams plans to give a speech on Thursday outlining a mental health agenda that calls for expanding key services for people with severe mental illness.

There are around 100,000 adults in the city with severe mental illness that is untreated, city officials say — people with conditions such as schizophrenia, bipolar disorder and major depression. There are thousands more whose care is slipshod or inconsistent.

The reasons for the gap are many, and include a lack of available psychiatrists, poor coordination between hospitals and other care providers, and the use of jails as de facto (and woefully inadequate) psychiatric facilities.

Mr. Adams’s plan includes:

• Creating a “single-access system” to connect people with severe mental illness to care in the community no matter where they live or where they are coming from, including homeless shelters, hospitals and jails.

• Fielding enough “mobile treatment” teams — clinicians who help people on the streets, and in shelters and jails — to serve 800 more people.

• Expanding a program that sends clinicians rather than police officers to respond to mental-health 911 calls and that currently operates in only a few neighborhoods.

• Increasing the capacity of “clubhouses” — centers of community for people with mental illness, who often suffer in isolation, that connect them with education and job opportunities and foster friendships.

“For people with serious mental illness, help doesn’t mean a single intervention,” Mr. Adams said in a statement. “Community is critical. This plan invests in community-based care, health care and supportive housing for New Yorkers with serious mental illness, strengthening the entire system that supports them.”

The plan — the latest in a series of similar measures by the mayor and Gov. Kathy Hochul — comes as the city approaches the third anniversary of the coronavirus pandemic, a period when many New Yorkers have struggled psychologically, and overdose deaths and homelessness have surged.

Mr. Adams, a Democrat who ran for office on a public safety message, announced a plan last fall to move homeless people with severe mental illness who pose a threat to themselves or to others off the streets and into hospitals, against their will if necessary. Mr. Adams has received both praise and fierce pushback to his sweeps of homeless encampments, his deployment of more officers in the subway and the involuntary removal policy.

In January, Ms. Hochul, also a Democrat, said she would compel hospitals to reopen hundreds of psychiatric beds that were closed during the pandemic, require hospitals to keep psychiatric patients until they are stable and coordinate discharge planning with community providers, and create new mobile treatment teams to serve more than 2,000 people in New York City.

The mayor’s new plan is short on details of implementation and cost. Mostly, it lays out a set of priorities; the city will need to figure out how to cobble together funding for most of them.

It is a companion to the policy that the mayor announced in the fall, which concentrated on how the city would deal with people who had completely unraveled, in public. That plan was criticized in some quarters as being heavy-handed and coercive, though it attracted plenty of support, too. The new plan, which focuses entirely on treatment that people can access voluntarily, addresses how to prevent them from falling apart in the first place.

“We want to really focus,” the city’s health commissioner, Ashwin Vasan, said, “on what is it going to take to ensure that as many people as possible who are at risk of falling into crisis never end up in that position and never end up needing the extreme acute care that we often have to provide.”

Doing this, he said, 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 will lay 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.

“I think the resources are well placed in terms of where the greatest acuity of need is,” she said. “When you look at this plan paired with the governor’s plan, it’s just clear that there’s an unprecedented focus on mental health.”

Brandon Jackson, 32, a client of one of the institute’s I.M.T. teams whose history includes jail for a violent robbery and hospitalization for creating disturbances at a shelter, said that over the past year, the team had helped get him on a track toward stability.

“With medical, with schooling, anything community-wise I need,” he said. “If I need food, if I need clothes, if I need to find a safe haven to go at, they support all around.”

He is now receiving monthly injections of antipsychotic medication from the team and is on a waiting list to move into supportive housing, which comes with on-site social services.