Last two season's top scorer Ranti Martins (65th) gave the lead with a header, while ONGC recruit Eric Brown (75th) struck a stunner after a goalless first-half to give United SC full three points to start the season.
After their title sponsors disappeared in the Bengal financial scam, the Kolkata team failed to woo a new sponsor to put together the team.
But the players including high-profile Ranti and Dutch coach Eelco Schattorie remained united to their club as they had only 21 days of practice to start the season.
It was a slow start to the match as both teams opted to play out patiently for a drab goalless first-half.
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A cautious United SC relied on short passes without being too adventurous, even as the Shillong outfit looked to attack.
Rangdajied got the first shot at the target in the 16th minute when captain Eugeneson Lyngdoh took chance of a defensive lapse for a low volley but was easily held on by Sangram Mukherjee.
On the rebound, C K Vineeth took a powerful shot, but it was Payar who averted an early mishap, even as the United SC players protested that the ball had crossed the goalline only to be ignored by the referee.
Minutes before the break, the RUFC wasted a sitter when an unmarked Edmar Figueira missed the target with a wayward header.
The home side looked more penetrating in the second-half resulting in goal in the 65th minute when the ace Nigerian headed the ball home from an Eric freekick.