In-form India striker Jeje Lalpekhlua distinguished himself with a fine brace and may well have completed his hat-trick had he accepted the chances he got at the Barasat stadium. Trinidadian striker Cornell Glen, midfielder Bikramjit Singh and substitute Azharuddin Mallick completed the lopsided scoreline for the record 13-time tournament champions.
Lajong came into the contest with an impressive win over Mohun Bagan's arch-rivals East Bengal. But the attacking might of Mohun Bagan's trio - Jeje, Sony Norde and Glen - who scored seven goals against Salgaocar in the two-legged quarterfinal was just too much for them.
After an even first quarter of exchanges, Mohun Bagan began to assert their supremacy and went into the break with a healthy 2-0 scoreline.
Jeje maintained his rich vein of form to put the hosts ahead in the 40th minute. Much of the credit goes to the Haiti international Norde for the goal.
Bikramjit began the move and released Norde through the middle, who swiftly took two defenders in his stride, before unleashing a venomous right-footer.
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Lajong keeper Vishal Kaith could only palm it up in the order and Jeje reacted quickest to put the rebound into the net.
The lead gave Mohun Bagan renewed impetus to pour forward in search of goals. And they found their target again five minutes later, just at the stroke of half-time.
This time, Jeje started the move with a neat give-and-go with Glen on the edge of the box and released the ball on the vacant left side of the box.
An onrushing Bikramjit blasted his effort on the roof of the net, the ball hitting the inside of the crossbar before nestling in, to swell the lead.
There was no respite for Lajong even after the break as they went 0-3 down five minutes after the restart.
Glen was on target this time, putting the final touch to a well-worked move from close range.
Lajong were helpless in front of such a scintillating offensive display and Jeje capitalised on their haplessness to make it 4-0 with a routine tap-in in the 56th.
With the fate of the contest a foregone conclusion even before the hour mark, Lajong needed away goals to keep alive the tie ahead of hosting the return leg on May 14.
They went forward, leaving gaps at the back and knowing that they could concede any moment. Lajong forced several corners but never looked like scoring.
Mohun Bagan stifled the flow of the game but not before punching a final nail into Lajong.
Substitute Azharuddin Mallick blew a regulation finish but seconds later atoned by coolly finishing Norde's cut-back pass inside the box from the left on the 86th.