Ethan Robson’s 72nd minute strike was enough to hand the hosts all three-points at Stadium MK, despite Pompey having the better of the second half.
The first chance of the afternoon fell to the hosts as Scott Twine found space down the left-hand-side and squared the ball to Troy Parrott, who was able is able turn inside the box and fire towards Gavin Bazunu, who stood tall and palmed his effort to safety.
Pompey then grew into the contest and should have taken the lead midway through the first half when Kieron Freeman collected the ball by the right touchline and swung an inviting cross towards the back-post, before Reeco Hackett-Fairchild found space but headed the ball into the ground that saw it loop over the bar from close range.
Following the 23-year-old’s missed opportunity, the hosts reclaimed the ascendancy and would have drawn first blood had it not have been for the woodwork or the brilliance of Bazunu. With 15 minutes remaining in the first half, Hiram Boateng saw his thunderous effort bounce off the foot of the post after he did well top open up his body and let-fly from the edge of the box.
Bazunu was then called into action shortly after when Twine drove towards the Pompey backline and shifted the ball to Parrott, who had found space in the box to curl an effort goalward that was matched by quick diving save and strong hand from the 19-year-old.
The second half started in similar fashion to the first, with both teams cancelling eachother out. The first clear-cut chance of the second 45 fell to John Marquis in the 60th minute when he had beaten the offside trap and did well to force Andrew Fisher into a smart save from an acute angle.
Pompey then gained control and tested Fisher again shortly after when Ronan Curtis finished a direct run into the box by striking towards the near post but saw the Dons keeper stand his ground and save well.
The Blues were by far and away the better team in the second half, but were caught on the counter attack with 20 minutes remaining as Twine collected a cross-field ball over the top of the Pompey defence, and glided his way into the box before squaring to Robson who tapped in from close range.
MK Dons have now leapfrogged Pompey into fourth in the League One table.