Ross Draper's free header from a 13th-minute set-piece sent Caley Thistle on their way and, apart from one first-half chance for Leigh Griffiths, Hibs never looked like getting back into the game.
Caley Thistle stepped up a gear after a dull first half and Richie Foran and Billy McKay secured their deserved win in the final 14 minutes as they stayed three points behind leaders Celtic.
The result came just six days after Hibs had recorded a superb William Hill Scottish Cup triumph over Hearts, but Fenlon had no such cause for celebration this time.
"It was a poor performance all round from start to finish," he said. "We defended poorly, which we haven't been doing.
"We didn't get to the levels of previous performances.
"It's hard to explain why straight after the game, we will need to analyse it.
"But when you are not playing well you have to make sure you stay in the game and do enough, but we let in sloppy goals."