Authorities say the child was wandering unattended in the Dumont area just after midnight.
Responding officers learned that the child's mother lived in the complex, and the child's aunt reported that her sister was not answering her phone.
Officers entered the mother's apartment, where they discovered the 36-year-old woman dead with stab wounds to the neck.
Soon thereafter, Closter Police located the victim's 36-year-old boyfriend walking in Closter with the victim's two other children, ages 18 months and 6 months.
The boyfriend was taken into custody but has not yet been charged.
WATCH: NewsCopter 7 over the apartment complex
According to a neighbor, there was constant friction between the victim and her boyfriend.
Police said there were no signs the victim put up a fight.
The 5-year-old child, from a previous relationship, was apparently left in the apartment with the victim, while the suspect left with the other two.
An investigation is being conducted by the Bergen County Prosecutor's Office Major Crimes Unit under the direction of Chief Robert Anzilotti, the Dumont Police Department under the direction of Chief Michael Conner, and the Bergen County Sheriff's Office under the direction of Sheriff Anthony Cureton.