Joseph Ponte, New York’s newly appointed correction commissioner, and his wife were visiting the city two weeks ago, and while she went apartment hunting, he had a few hours free. He dropped by Rikers Island to have a look at the jail block where a mentally ill inmate died this year, left unattended in a stiflingly hot cell.

“I wanted to see it for myself,” Mr. Ponte said.

The death, of course, would soon be his problem, along with so many others at Rikers, the country’s second-largest jail. Mr. Ponte, who starts his job on Monday, spent the last three years in Maine remaking the state’s prison system by reducing the use of solitary confinement, overhauling mental health care and welcoming advocates and reporters into the prisons.

In Maine, as correction commissioner, Mr. Ponte oversaw a population of 2,000 that was small enough to micromanage: No inmate could be held in solitary confinement for more than 72 hours without his approval.

But even for Mr. Ponte, 67, who has built a national reputation as a reformer, Rikers Island, with 12,000 inmates, is not likely to be so easily tamed.