The incident happened just after midnight Monday at their home on 27th Street near 38th Avenue in Long Island City.
Louise Herminda, 75, was planning to return the dog, a mastiff, Monday because it was too aggressive and uncontrollable, neighbors said. But she apparently waited too long.
The dog attacked Herminda, causing what police described as trauma to her upper torso area. She was pronounced dead at New York Presbyterian Hospital at around 6 a.m.
Herminda's 39-year-old son, who has autism, was also mauled in the attack. He suffered non-life threatening injuries to his lower-left leg.
Neighbor John Brien said the same dog attacked a neighbor's dog 10 days ago while being walked on a leash. He said the mastiff went after a small terrier and wouldn't let go.
"He just pounced on it and started chewing it like a rag doll," Brien said. "It started ripping it to bits. We tried to jump in, the owner of the white dog got bit on the hand."
Herminda's next-door neighbor, Rosa Ortiz, said that attack was what led Herminda to decide to get rid of the dog.
She said she was the first one to get to the house after the attack, and described the scene as horrifying.
"When I saw her there, it broke my heart," Ortiz said. "She was all bloodied. Her head, her arms. It was really bad. It was a tragedy."
The dog was sedated by NYPD ESU officers and is now in the custody of Animal Care and Control of New York City.
Animal Care and Control said the dog was adopted from them almost 6 years ago when it was about 1-year-old at the time. The dog is currently at ACC's Manhattan Care Center.
Neighbors said the mastiff was one of up to 10 dogs living in the basement of the home.
"When you put a large group of dogs together they have to be watched very carefully to make sure they aren't attacking one another and they really need to be under proper supervision," said Jean Shafiroff, an animal advocate and ambassador with the American Humane Society.