Trailing heading into the 88th minute, Alex Gilbey fired in against his former club to equalise before Dennis scored in the 99th minute on his first appearance of the season to secure the win.
Mo Eisa opened the scoring after just eight minutes with his fourth goal in as many games, but Joe Taylor scored twice in three minutes to see the U’s lead as of the 13th minute.
Here’s how we rated the players at the JobServe Community Stadium.
1. How MK Dons rated against Colchester United
Two late goals rescued victory from the jaws of defeat for MK Dons. See who rated Top Don in our rankings at the JobServe Community Stadium Photo: Jane Russell
2. Craig MacGillivray - 7
Should point his finger at his defenders for both goals when Taylor struck twice in quick succession, but recovered well with some decent saves to keep Dons in the game Photo: Jane Russell
3. Warren O'Hora - 7.5
Dons' best defender on the day. A voice of reason and calm on the pitch and pushed into the midfield at times too to add extra bodies forward Photo: Jane Russell
4. Jack Tucker - 7
Generally pretty solid, recovered well after Dons' slow start. His header hit the bar in stoppage time as Dons pushed on to win it Photo: Jane Russell