Skipper Katsumi Yusa's 9th minute header from long throw-in by Dhanachandra Singh opened the floodgates before Cornell Glen (28th minute), Luciano Sabrosa (33rd) and Balwant Singh (48th) completed the rout as the green and maroon brigade braced themselves for the derby against arch-rivals East Bengal in the coming round.
However Bagan's defensive worries remained as the struggling Goans pulled back through Darryl Duffy (69th) and Gurjinder Kumar (86th) but the that was too little too late in the end.
The defending champions climbed to the top of the table pushing Bengaluru FC to second spot on goal difference as both have six points from two matches while Salgaocar remained without any point.
Mohun Bagan doubled the lead after Glen was brought down by Gurjinder Kumar inside the box as the Trinidad and Tobago World Cupper made no mistake from the spot.
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Making his debut in green and maroon colours, Brazilian Sabrosa tripled the lead within five minutes from a corner with his brilliant header from an acute angle which caught the Salgaocar defence napping.
Two more chances went abegging just before the break but it did not matter much as Mohun Bagan returned after the changeover to score a 4-0 lead after Balwant placed his head to perfection from a Katsumi corner.
Jeje Lalpekhula was brought in the 67th minute in place of Balwant but that did not yield much and the Mizoram set up Glen but his shot flew over the post.
In the meantime, Duffy pushed Jackichand Singh's diagonal chip past an advancing Mohun Bagan goalkeeper Debjit Majumder for to pull one back in what the crowd protested for an off-side.
Smarting from their 1-2 defeat to former champions Bengaluru FC in the opening leg, Salgaocar coach Malky Thomson named Leny Pereira in place of Nicolas Colaco in the only change to their starting line-up.
The I-League winning coach Sanjoy Sen made a host of changes to the line-up that beat Aizawl 3-1 last week.
Debjit Majumder, Azharuddin Mallick and India international Pronay Halder were inducted in the starting XI while Brazilian Luciano Sabrosa in charge of deep defence with star striker Jeje brought in as a substitute.