Mullin got things going from the penalty spot, converting a Staggies spot kick after Alan Newlands said Lewis Ferguson handled the ball in the box.
The Dons hit back in style though, with Niall McGinn equalising just six minutes after Mullin’s opener. Second half goals from Ryan Hedges and Andy Considine sealed the win for the away side.
County remain in the top six for now, but Mullin says his side can’t afford to lose goals from set pieces so easily against a ‘great’ team like Aberdeen.
He said: “I think we got off to a good start and I thought we had an excellent 45 minutes. We frustrated them and had a couple of chances, so it was a good start.
“It was frustrating to concede from two free kicks. We don’t point fingers during the game but we’ll analyse what went wrong at those set pieces. We still felt strong and were pushing to get a second goal but it’s hard as Aberdeen are a great side and know how to manage these games.
“Everyone knows the number of goals we’ve conceded in the last seven games and it’s not good enough. That’s not just defensively though, that’s collectively as a unit, because we set up to defend from the front.
“But we won’t go pointing fingers, we’ll be working on it. Maybe some people were tired and when you concede sloppy goals it does deflate you but we kept pushing.”