Mr. Garner’s mother, Gwen Carr, said that she was “outraged and disgusted” to learn of Sergeant Adonis’s plea deal and accused Mayor Bill de Blasio and the Police Department of “actively participating in an ongoing cover-up.”

“They don’t want the public to know how deep, how wide and how high the wrongdoing in this case went,” she said. “Their actions are disgraceful, but I am not going to back down. I will continue to fight.”

Sergeant Adonis, a 17-year veteran, could not be reached for comment. Her labor union, the Sergeants Benevolent Association, scheduled a news conference on Thursday.

“We will address what really occurred in this extremely sensitive matter,” said Edward D. Mullins, the union president, who has said the decision to charge her was politically motivated.

Mr. Mullins has said that Sergeant Adonis did not stand idly by after she heard Mr. Garner say he could not breathe. Instead, he said, she followed department protocol and consulted with another officer at the scene, a trained paramedic.

Sergeant Adonis was supposed to be in a meeting but responded to the scene on her own initiative, Mr. Mullins has said. Other supervisors — the duty captain, the borough commander and the zone commander — were never reprimanded.

Sergeant Adonis had been promoted to sergeant in June 2014 and was assigned to the 120th Precinct in Staten Island on July 9, eight days before the encounter. Until then, her record had been unblemished and she had received positive evaluations, Mr. Mullins and police officials have said.