Born in Suffolk, Upson began his career with Ipswich Town and, at the age of 15, helped the Tractor Boys lift the 2005 FA Youth Cup by scoring an extra-time winner in the final.
Upson signed his first professional contract in April 2008 but made just two appearances for the Blues before joining League One side Yeovil Town.
He scored 17 goals in just under 150 games during his time at Huish Park and in 2012/13 helped the Glovers achieve a remarkable promotion to the Championship via the Play-Offs.
Mid-way through the 2014/15 season, he joined Millwall for an undisclosed fee and in his two-and-a-half years at the New Den, he played over 75 times.
Despite helping the Lions reach the League One Play-Offs last season, Upson left the London club this past summer but was signed up by the Dons just four days later.