Lajong rode on a Dipanda Dicka brace to take the match away from the visitors in the first half itself, who slumped to their third straight defeat on the trot.
Dicka got the hosts lead in the 28th minute and followed it up with a second goal three minutes before half-time.
Karan Sawhney reduced the deficit for Mumbai in the 56th minute.
Chinglensana added Lajong's third in the 70th minute to seal the tie for the hosts.
The victory catapulted Lajong to the fifth spot, with six points feom five matches, while Mumbai are a spot behind having the same number of points but inferior goal difference.