Lajong started on an attacking note right from the start and poor defending from East Bengal saw goalkeeper Abhra Mondal coming out far ahead from his position at least twice in the first 10 minutes.
After goalless first half, Lajong showed urgency for the goal and in the 62nd minute Lynsingh slotted in after a good pass from Bademi. East Bengal came close on a couple of occasion in the last few minutes but Gumpe Rime's inspiring performance under the bar took Lajong into the semis.
Earlier, fresh from their IFA Shield final loss to Churchill Brothers, Mohun Bagan put on a clinical performance against ONGC.
The Green and Maroon brigade from Kolkata could have won by a bigger margin, but Jose Barreto missed a penalty. The Brazilian, however, atoned for it in the 43rd minute by scoring a brilliant goal in the group two match of the tournament.
The 16-time champions took went ahead in the ninth minute after an unmarked Surkumar hit from a rebound off James Singh's perfect flag-kick.
Nigerian Chidi came close to double the lead in the very next minute but missed the sitter after being set up by James. The Nigerian striker finally found the net six minutes later after he scored from a goalmouth melee to stun the hapless ONGC defenders.
In the last 10 minutes, Mohun Bagan were all over and Chidi completed his brace in the 82nd minute from Shanta Kumar's precise pass. Surkumar struck again in the dying minutes with a long ranger to complete the tally.