The girl called her 19-year-old boyfriend at home in Shankar Nagar locality of the city on July 2 and the boy allegedly stabbed the girl's mother, Gurwinder Kaur Birdi (40), with a knife in her abdomen and later strangled her to death, Badrakali police station inspector C A Baravkar said.
The duo took the body in the family car of the girl and dumped it at Kasara ghat, police said.
Later, the girl herself went to Badrakali police station to file a missing complaint and also informed her uncle (mother's brother) that her mother was missing.
In the meantime, the Kasara police of Thane district found a body of the woman at Kasara ghat bearing abdominal wounds and strangulation marks on the neck.
They coordinated with the Badrakali police station in Nashik after getting information that a missing person complaint was filed there.
Police then questioned the girl during which she confessed that she conspired with her boyfriend to kill her mother.
The girl alleged that she would have frequent fights with her mother as the latter was opposed to her friendship with the boy, police said.
Meanwhile, both the accused are under police detention and would be produced before court.