By his count, Francis J. Greenburger has built or owned more than 20,000 apartments over the past 50 years.

Starting out at age 19 with a five-story brick rental on Barrow Street, Mr. Greenburger built one of the city’s largest co-op conversion businesses in the 1970s and 80s, with projects that included the Delmonico Building, 1045 Park Avenue and the sprawling Clinton Hill Co-ops. Next came Midtown office towers, apartments from New Jersey to Berlin, and even the stray Nova Scotia outlet mall and Tallahassee, Fla., parking garage.

Yet for all of his 20-million-square-foot empire, the project Mr. Greenburger may be most excited about — certainly the one he is most determined to build — is a 25-bed center to treat convicts with mental illnesses.

“These aren’t criminals,” Mr. Greenburger said during an interview last week at his 15th floor office at 55 Fifth Avenue. “These are people who have committed crimes, mostly because they don’t know any better or they are acting out on impulse. And study after study has shown that prison only makes this behavior worse.”