In front of a packed 65000 crowd at the Vivekananda Yuva Bharati Krirangan, which was hosting its first match after grass was relaid, the green and maroon brigade had to return disheartened after the shock defeat.
East Bengal took their tally to an unbeatable 25 points from nine matches to seal the issue with one match to spare, while the I-League champions remained on 14 from eight matches to see their title hope go up in smoke.
In imperious form, the Korean signing scored a double from his signature freekick, this time from the top of the box, in the 36th minute to further dent Mohun Bagan's slim title hopes.
Down 0-2 and nothing going in their way, Mohun Bagan brought in their Japanese forward Katsumi Yusa after the changeover but it did not yield much as five minutes later another ace marksman Dudu Omagbemi was sent off, leaving them with 10 players.
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Mohammed Rafique's made it 3-0 from Bikash Jairu's left-footed cross in the 81st minute, while Rahul Beke struck from a corner kick by substitute Saumik Dey in the 90+3rd minute to complete the rout.
"It's great to be a part of the history... But our first title (in this era), in 2010, will always very special because it marked the beginning of the journey . But last season's title when we clinched it after beating Mohun Bagan 3-1 was the most challenging one," former captain and midfielder Harmanjyot Singh Khabra, who joined the side in 2009, said.
East Bengal's six-title winning streak had begun in 2010 under Englishman coach Trevor James Morgan, who left the club after guiding them to a hattrick of titles in 2012.
Under Marcos Falopa and Armando Colaco, East Bengal had annexed their fourth and fifth successive titles in 2013 and 2014, making it a record 36 in total.