As Black Lives Matter was growing into a social movement, the killing of Eric Garner by the police on Staten Island and the harsh treatment of teenage inmates at the Rikers Island jail complex pushed New York activists into a national spotlight.

One of them was Glenn E. Martin, an ex-convict who was sought out for his firsthand knowledge of the dangers and hope found behind bars. In interviews and in speeches, he recounted being stabbed by fellow inmates as a teenager at Rikers and earning a college degree as an adult in an upstate prison while serving a sentence for armed robbery.

In 2014, Mr. Martin had started a nonprofit advocacy group, JustLeadershipUSA, and over the next few years, he became a vocal proponent of closing Rikers, where pervasive abuse by guards and persistent violence among inmates were drawing new attention to the need to reform the city’s jail system. In 2016, Mr. Martin received standing ovations as he accepted the Robert F. Kennedy Human Rights Award in Washington.

As his reputation as a fund-raiser and advocate grew, so, too, did talk that he was using his professional prominence to pursue sexual relationships with women who were drawn to the criminal justice reform movement — and in some instances engaging in sexual misconduct.