When Joseph Ponte of Maine arrives for his first day on the job next week as the city's corrections commissioner, he'll step into an ugly crisis that's been brewing for years.

The city's jail system on Rikers Island has increasingly become the default lodging for New Yorkers with mental illnesses and homeless problems.

The results are nothing short of scandalous:

Officials are investigating the death last February of a 56-year-old ex-Marine with a history of mental illness and homelessness who may have been allowed to die in his cell when a damper and rooftop fan malfunctioned.

A Rikers Island supervisor was arrested last week on federal civil rights charges that accuse him of ignoring the pleas of subordinates and leaving a mentally ill inmate to die after the prisoner gulped a packet of toxic detergent.

And the trendlines are only getting worse:

The proportion of mentally ill inmates in the city's jail system has skyrocketed from 24 percent in fiscal 2007 to nearly 40 percent today, according to corrections figures.

Inmate-to-inmate violence resulting in serious injuries, meanwhile, jumped 21 percent between July and October of 2013 compared with the same period in 2012.

Ponte will arrive in the city with a solid reputation as a reformer. As commissioner of Maine's prison system, he scaled back the use of aggressive disciplinary measures and curbed the use of solitary confinement as punishment.

His first order of business in the city is to ensure that our jails can competently supervise an influx of mentally ill inmates that shows no sign of ending.

Then comes the hard part. New York is facing a perfect storm of failed policies -- not just for corrections -- but for homelessness, mental health and veteran services.

The jails should not be the last way station for the city's homeless mentally ill residents -- and the ultimate burden for the city's cascading failures shouldn't just fall on Ponte.

But with our safety nets in shreds, Ponte will need to provide basic services to those who need them while his boss -- Mayor Bill de Blasio -- builds a broader system of social services that works.