The match gained pace after Jeje broke the deadlock in the 40th minute before he set up Bikramjit Singh for a 2-0 lead at the half time.
The Mizoram striker returned with menacing form in the second half scoring two goals in the space of four minutes as Mohun Bagan toyed with Lajong defence with ease.
Mohun Bagan's U-22 player Azharuddin Mallick, who replaced Cornell Glen in the 61st minute, struck their fifth goal from a Sony Norde throw-in in front a jubilant 12,342 home supporters.
After hits and misses, Mohun Bagan finally got the opening when Bikramjit released the ball to Sony Norde who cut through two defenders to take a swipe from 20 yards out on target.
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Lajong goalkeeper Vishal Kaith came off with outstretched arms to block the shot but the loose ball was calmly deposited at the net by an unmarked Jeje to open his and team's account.
Five minutes later, Bikramjit found his name on the scoresheet. Jeje initiated the move to Cornell Glen who then flicked it to the left and a surging Bikramjit saw his shot hit the bar before getting into the net.