Darren Barr gave Hearts the lead on the stroke of half-time but Leigh Griffiths' superb leveller from a free-kick and Ross Caldwell's 90th-minute strike sealed the win which lifted Hibs above their rivals in the Clydesdale Bank Premier League table.
"To get a sucker punch like that so late on after playing so well and dominating the first half, the character they (the players) showed in the second half to play against them as well as we did was fantastic," said Fenlon.
"We played so well in the first half and had all of the chances and then a sucker-punch goal. That sometimes can knock the stuffing out of players but I said at half-time 'go and win the game'.
"Credit to every one of them, to a man, they were fantastic."
Fenlon is now looking ahead to the Scottish Cup final against Celtic on May 26.
He added: "We need to believe in ourselves and eradicate mistakes.
"It's a great day. I'm thrilled for the supporters. That's what it's about today as we haven't won here for a while against Hearts.
"We've turned the tide a little bit and that's more for them than for anyone else."