Mackay, who can operate at right-back or in central defence, arrived from Livingston last May, signing a two-year deal, but that has now been lengthened.
Saints paid £50,000 in a joint deal for Mackay and midfielder Murray Davidson.
And that looks like a shrewd investment, with Mackay featuring in all but two of the 45 games played by St Johnstone this season, while Davidson's performances earned him a call-up for Scotland at Under-21 level.
"I'm delighted to have got this extension to my contract done so quickly and I can enjoy my summer break now," Mackay told the club website.
"Last season worked out so much better than I ever could have imagined. I thought I might have to be content with being a squad player, so to get straight into the team and remain there for more or less the entire season was a real bonus."
"Last season was one of the best of my career and certainly the most enjoyable.
"The manager made me feel welcome from day one and along with the rest of the lads. What I want to do now is try and build on what we achieved last season. There is no reason why a top six finish isn't a realistic aim."
Mackay played at Dundee and Oxford before moving back to Scotland with Livi in 2005.
Source: BBC Sport
Source: BBC Sport