The Edinburgh club looked set to hold on for the win at Tannadice after John Rankin's early opener was cancelled out by James McPake and a Leigh Griffiths strike put the visitors 2-1 ahead.
But United claimed a share of the points when Johnny Russell converted from 12 yards with just four minutes to go after Ryan McGivern fouled Gary Mackay-Steven.
Television replays showed the challenge took place outside the box but Fenlon was reluctant to be too hard on the match official.
He said: "I've just watched it again and it's outside the box but these things happen. We all make mistakes and we've just got to get on with it. The game is so quick.
"It's a good foot outside the box but we've just got to move on. It's important we take the positives out of it and not look at a mistake from the ref or whatever else."
The late leveller was the difference between Hibs moving up to fourth spot in the Clydesdale Bank Premier League - and level on points with third-placed Motherwell - and having to settle for sixth position instead. Fenlon added: "We can't do any more as a team than we did.
"We created chances, we played really well and we did enough to win the game. We defended well and it's outside of our control when things like that happen but what can we do?
"I haven't spoken to the referee. I'm not one that goes banging on refs' doors. It's a hard enough job without me doing that. You hope they balance themselves out over time.
"I'm disappointed that we didn't pick up three points but I'm chuffed with the performance. I thought we were very good and I can't take anything away from the performance of the players."