Jennie Mattison, 23, pleaded to one count of felony 2nd-Degree Hindering Prosecution in the death of her son, Gary Carpenter, in November 2012.
Police say the boy was the victim of assault and died from his injuries because she did not seek treatment for him for 12 hours.
Officers responded to the couple's Glens Falls home, finding the boy not breathing. He was transported to Glens Falls hospital, and from there, air-lifted to Albany Medical center where he later died from a traumatic head injury.
Brandon Warrington, 24, Mattison's boyfriend and not Gary's father, faces charges of Murder in the 2nd-Degree, Manslaughter 2nd-Degree and Endangering the Welfare of a Child. He previously pleaded not guilty to all charges.
Authorities say Warrington knocked the boy off his feet, causing a head injury, then hit him again when he wouldn't stop crying. They say his injuries indicate this was not the first time he had been assaulted.
Warrington and Mattison have one child together, a 3-month-old baby.