Well made a bright start to the game as they looked to gain their first win of the season and they could have been ahead in the second minute, but Scott McDonald's header rebounded off the top of the bar and fell clear.
The Well continued to push forward and both David Clarkson and Phil O'Donnell's had attempts at goa,l but they did not trouble Jamie Langfield in the visitors goal. In the opening exchanges the Dons had shown little to trouble the home side.
This all changed on the 33rd minute. A long ball from Barry Nicholson looked as if it was being dealt with by Graeme Smith, but the keeper misjudged the flight of the ball and this allowed Mackie to nip in and slide the ball into the empty net from a tight angle.
The home side kept pushing forward and just before the break should have scored an equaliser when McDonald and David Clarkson carved the Dons defence open, only for the striker to slice the ball wide when clear on goal.
Both sides had early chances after the break with Karim Touzani coming close for the Dons and O'Donnell volleying wide for Well.
Then on the hour mark the Dons struck again when a corner was swung into the six-yard box and keeper Smith appeared to be barged as went for the ball and Mackie was on hand to bundle the ball over the line.
Despite committing four men forward Motherwell still couldn't find a way through a resolute Dons defence and slumped to their third defeat in a row, while the Dons recorded their first win and remain undefeated.