The 54-year-old actor is believed to have committed suicide by hanging himself at his residence at Purasawalkam here, they said.
The actor's wife found his body hanging through a window after repeated attempts to open the door locked from inside, police added.
No suicide note has been found so far.
Mohan, who acted in "Boys" and "Allithandha Vaanam" is survived by his wife and his son, who is presently settled in the United States.
Police is investigating into the incident.