Grant left Rochdale to move to Milton Keynes for an undisclosed fee, the club have confirmed.
The 20-year-old was well-thought-of at Wednesday but left Hillsborough in search of first team football in January last year under the watch of then interim Owls boss Neil Thompson.
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The Republic of Ireland under 21 international made just one appearance for Wednesday, in a League Cup tie against Rochdale, who he would then go on to join.
Dale were relegated to League Two in Grant’s first season there but impressive performances last season saw the Dublin native draw interest from a number of EFL clubs.
MK Dons, who like Wednesday, lost in the play-off semi-finals last season have won the race to sign Grant as they too look to make another push for promotion this coming season.
Boss Liam Manning told the club’s website: “Conor is an exciting addition to the squad and a player we are looking forward to working with.
“He has a number of qualities, technically and physically, which we have seen from watching him regularly at Rochdale. He has shown good potential so far and it is now down to him, with the support of the staff, to unlock that promise and produce high-quality moments on a consistent basis.”
Grant becomes MK Don’s fifth signing of the summer having already snapped up Matt Dennis, Ethan Robson, Jack Tucker and Nathan Holland.