Authorities say the 48-year-old victim was attacked inside her apartment on Beach 96th Street in Far Rockaway sometime after 4 a.m. Thursday.
She was stabbed in the abdomen and rushed to Peninsula General Hospital, where she was pronounced dead.
The ex-boyfriend then allegedly assaulted her two teenage daughters, ages 15 and 16. The 15-year-old was reportedly slashed in the face. The 16-year-old is pregnant.
Both were rushed to Peninsula General Hospital, where they were treated and released.
Detectives are looking for the 53-year-old suspect, identified as Charlie Mitchell.
Mitchell moved out of the victim's apartment one month ago.