Joaquim Abranches and Ryuji Sueoka scored the two first half goals for Dempo in the 40th and 45th minute respectively, while Godwin Franco scored in the 70th minute to complete the three-goal tally after Mohun Bagan were reduced to 10 men when Biswajit Saha was sent off in the 53rd minute.
Armando Colaco continued with Ryuji Sueoka and Joaquim Abranches as his two strikers, with Godwin Franco in place of suspended Climax Lawrence while Subasish Roychowdhury was back in the goal.
Karim Bencherifa, in his first match in Goa after leaving Salgaocar, started with Shilton Paul between the sticks while Odafa Onyeka Okolie played in front of Cyprian Stanley.
The match started at a brisk pace with Dempo midfielder Godwin Franco hitting the top right corner of the woodwork from a third minute free-kick while the visitor's captain shot wide on a couple of occasion.
Mohun Bagan followed up with two wide shots from Sabeeth Sathyan and Biswajit Saha on the right and left sides of the pitch respectively in the 10th minute, though both the clubs found it difficult to make inroads through their opposition's defence, with both of them being very organised at the back.
The only sort of exception was when Odafa toiled against the rival defense in the 35th minute before slipping a pass to Lalrin Fela, who tapped goalwards, but the attempt was scrappily saved by Dempo goalkeeper Subasish Roychowdhury.
In the 40th minute, finally came the breakthrough after Samir Naik's pass found Romeo Fernandes on the right wing. The Dempo midfielder squeezed a cross in between the two center backs of Mohun Bagan where Abranches did the rest by tapping home.
Four minutes later, Dempo were awarded a penalty after Peter Carvalho was brought down in the box by Nirmal Chettri. Sueoka, who had initiated the attack with Romeo, took the honour of scoring from the spot in the 45th minute.
As the second half resumed, Dempo immediately started pressurising the Mohun Bagan defense with an aim to prevent the oppositions from finding their footing in the match, though Odafa did force a save off substitute Dempo keeper Laxmikant Kattimani in the 52nd minute.
The away side's midfielder Biswajit Saha then went on to pick his second yellow card of the evening for a tackle on Godwin Franco, deemed dangerous by Karnataka-based referee B. Balasubramaniam, and as a result had to walk off the pitch.
Following the hour mark, the crowd was treated by some artistic footwork by former Premier League player Rohan Ricketts, who instantaneously provided an impact after being brought on in place of Miranda, as he forced a save from Mohun Bagan keeper Shilton Paul after being set-up by Sueoka.
In fact, it was from Ricketts' free-kick that was cleared by Nirmal Chettri, but only to land at the feet of Godwin Franco, who made amends after missing out early in the first half to score from about 25 yards out in the 70th minute.
The last few minutes of the game passed off without any scoring incident, as Dempo registered a win that sees them move to just a point off leaders East Bengal on the table, while the Mariners languish nine points off their city rivals.
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