The win propelled the Bangalore outfit to the top of the table with seven points from three matches.
On the other hand, this was United SC's first loss in three matches after the Kolkata side had won their first two games at home.
It was in the fourth minute of the match when man-of-the-match Johnson headed in a perfect corner by Beikhokhei Beingaichho to register his second goal in the ongoing I-League.
The hosts were the dominant side right from the kick-off forcing the visitors to rely more on counter attacks.
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The hosts could have doubled their lead in the 18th minute but Keegan Perreira failed to convert from close range.
Thereafter unmarked Sean Rooney also missed a sitter as he headed the ball straight to rival goalkeeper Sangram Mukherjee.
In between, United SC had their chances too. In the 24th minute, Ranti failed to slot it past Bengaluru FC goalkeeper Pawan Kumar and 13 minutes later, he shot it wide from inside the box.
United SC tried to regroup. They did enjoy possession but failed to penetrate the Bengaluru defence. In the 80th minute, United coach Eelco Schattorie was sent-off by the referee for constantly infringing upon he technical area.
Bengaluru FC will next face Kolkata's Mohammedan Sporting at home on October 10.