The Scottish Football Association's judicial panel rescinded the red card handed out to the Saints skipper during the second half of the 3-0 home defeat earlier this month.
Mackay was adjudged to have fouled Dutch winger Derk Boerrigter in the box resulting in a spot-kick converted by Nir Biton which put the champions 2-0 ahead.
Manager Tommy Wright was furious at the award of the penalty, and his full-back's dismissal, and lodged an immediate appeal.
Boerrigter was subsequently hit with a two-match ban for 'simulation' by the SFA compliance officer, who later found in Mackay's favour and formally overturned referee John Beaton's original decision.
The panel's verdict allows Mackay to take his place in the Saints' backline against Aberdeen at McDiarmid Park this Saturday (3.00pm).