Robert Dennison, a former chairman of the New York State Parole Board who became a powerful champion of wrongfully convicted or remorseful inmates in high-profile cases, died on July 26 at a hospital in Manhattan. He was 75.

The cause was complications of leukemia, his son, Robert Dennison III, said.

Mr. Dennison’s post-retirement role on behalf of convicted criminals was all the more significant given the parole board’s wariness in dealing with inmates accused of brutal crimes, especially against law enforcement officers. And it was surprising given his own political credentials: He had enrolled as a member of the Conservative Party to improve his prospects of being appointed to the board in 2000.

Several years after he retired from the board in 2007, he enlisted in the campaign to overturn the conviction of Johnny Hincapie, who was charged in the gang attack on a family of tourists from Utah on a Manhattan subway platform in 1990. That attack, in which 22-year-old Brian Watkins was stabbed to death protecting his mother, provoked citywide outrage, encapsulated in the tabloid headline “Dave Do Something,” beseeching Mayor David N. Dinkins to crack down on escalating crime.

“Nobody puts Johnny on the platform,” Mr. Dennison said of the witnesses who emerged years after Mr. Hincapie had been convicted, although he said his confession had been coerced. He was eligible for parole in 2013 after having earned a master’s degree in theology. He had been imprisoned for 25 years when the verdict was voided in 2015. Prosecutors decided not to retry him.

His conviction was overturned thanks to appeals by Mr. Dennison and Mr. Hincapie’s lawyer, Ron Kuby, and to the investigative efforts of William Hughes, a journalist who is now a professor at York College in Queens.

“Bob met Hincapie in Sing Sing at a graduation ceremony,” Mr. Hughes recalled in an email. “Hincapie gave him a copy of an article I published about his case in 2010, then Bob called me and offered to help track down potential witnesses. Bob was instrumental in getting the key witness to testify and in getting Kuby to take it.”

Mr. Dennison’s pleas on behalf of prisoners seeking parole, pardons and commutations of their lengthy sentences were all the more noteworthy because he had been appointed chairman in 2004 by Gov. George E. Pataki, a Republican, who reassigned Mr. Dennison’s predecessor, Brion D. Travis, after the governor took heat for the parole board’s 2003 decision to release the former 1960s militant Kathy Boudin.

Ms. Boudin had served 22 years after pleading guilty to murder and robbery in the bungled 1981 heist of a Brink’s truck by members of the Black Liberation Army and a Weather Underground offshoot, which left a security guard and two police officers dead.

In 2017 Gov. Andrew M. Cuomo, a Democrat, was under pressure from progressives to free Judith Clark, who had been the getaway driver in the robbery. Among those lobbying on her behalf was Mr. Dennison.

Mr. Cuomo commuted Ms. Clark’s sentence, which meant she would be eligible for parole immediately instead of in 2056, when she would be 107. After initially being denied parole, she was released in 2019.

In a 2010 letter to David A. Paterson, then the governor, Mr. Dennison described Ms. Clark as “the most worthy candidate for clemency that I’ve ever seen” and added, “If you look at what she did and all the deals the government made with other people involved, she sort of got left holding the bag.”

Mr. Dennison began advocating on behalf of inmates as a volunteer after retiring from the board.

“Bob became an advocate for all prison inmates — not just those claiming wrongful conviction — and a vocal critic of the arbitrary and arcane parole system,” Mr. Hughes said. “He spent much of his retirement visiting prisons, talking with inmates and writing letters to the parole board.”

Mr. Dennison had grown disillusioned with the parole board’s perfunctory review process, which typically consisted of two or three commissioners spending 15 minutes at most interviewing an inmate from afar by closed-circuit video after reviewing the case file. He also said that board members were often reluctant to release inmates in controversial cases.

“It is an easy job if you don’t have courage and you don’t have compassion,” he told The New York Times in 2010. “Because then you really don’t care. And then it is easy to make whatever decision you want without feeling guilty, without feeling, ‘Gee, maybe I made the wrong decision.’”

Robert John Dennison Jr. was born on July 18, 1946, in the Bronx. His father was a salesman. His mother, Catherine (McGann) Dennison, was a homemaker.

After graduating from Archbishop Stepinac High School in White Plains, N.Y., he received a bachelor’s degree from Iona College in New Rochelle in 1968 and a master’s degree in counseling from Iona in 1971.

His first wife, Mary Ann (Groh) Dennison, died in 2010. In addition to their son, he is survived by his wife, Maria (Gil) Dennison; two daughters, Ann Walsh and Elizabeth Flandreau; his sister, Maryann Riordan; his brothers, Joseph and Chris Dennison; and six grandchildren.

Mr. Dennison, who lived in Westchester County, was hired by the state as a parole officer in 1974 and went on to spend more than three decades in correctional services. He worked his way up to assistant regional director of parole operations, overseeing Brooklyn and Queens, before being named to the parole board. Board members are appointed by the governor, typically at the recommendation of a local state senator.

Mr. Dennison acknowledged that working with inmates and former prisoners had not been his first choice as a profession. He originally tried teaching sixth grade students, he said, but he found that was “too hard a job for me.”