“Norman is a tyrant,” said William Valentin, who spent five years on the executive board and was kicked out by Mr. Seabrook in August 2015. “The executive board is pretty much under his control. They really don’t argue with him too much. Whatever he says goes.”

Understanding Mr. Seabrook’s outsized importance on the cellblocks requires understanding the history of the city’s Correction Department. By the end of the administration of Mayor Michael R. Bloomberg, a Republican turned political independent, there was a real power vacuum in the department. It was considered a low priority, and the commissioner at the time, Dora B. Schriro, was a weak leader.

Mr. Seabrook stepped into that void, his power perhaps reaching its apex in fall 2013, when he almost single-handedly shut down the city court system by directing his members in a work stoppage that halted almost all of the buses that ferry inmates to and from court.

A judge complained that the court system had been “held hostage,” and Mr. Bloomberg sued the union. But Mr. Seabrook emerged unscathed.

Even after Mr. de Blasio took office in 2014 and appointed the reform-minded Joseph Ponte as correction commissioner, Mr. Seabrook continued to behave as if he were in charge of Rikers. He called a news conference in which he derided Mr. Ponte as a “hug a thug” yokel from Maine who was out of his league.

Mr. de Blasio seemed to go out of his way early on in his administration to try to cultivate the union leader. During the height of the Ebola crisis in late 2014, for example, the mayor took a break from emergency preparations to attend a charity dinner hosted by Mr. Seabrook at a Bronx ballroom. In a speech, Mr. de Blasio described him as “a friend” and “a great leader in this town.”

But the landscape was changing. News organizations and city investigators were exposing a culture of pervasive brutality in the jails. Preet Bharara, the United States attorney for the Southern District of New York, whose office filed charges against Mr. Seabrook on Wednesday, eventually joined a class-action lawsuit that led to intervention by a federal monitor. And the new mayor took an aggressive stance, vowing to remake Rikers.