The deal ties the Northern Ireland international to McDiarmid Park for next season, which will be his fourth campaign in Perth.
Defender Frazer Wright extended his deal earlier this week, joining Brian Easton, Gary McDonald, Stevie May, Scott Brown and Tam Scobbie in being contracted to the club for next term.
Meanwhile, St Johnstone manager Tommy Wright believes the defensive qualities of Inverness, who visit McDiarmid Park on Saturday, are underrated.
Caley Thistle have only conceded 21 Scottish Premiership goals this season, the lowest total behind Celtic.
While the prolific goalscoring of Billy McKay is the highest-profile feature of a hard-working team, Wright feels the defensive aspect is crucial to the League Cup finalists’ recent success.
Wright said: “I think what gets overlooked, because McKay gets the headlines, is that they are very strong defensively. The partnership of (Josh) Meekings and (Gary) Warren at centre-half is as good as any in the league.
“They are a good side and they have maintained a good consistent level of form throughout the season.”
Sixth-placed Saints, who are in the midst of a run of five consecutive league games against the teams above them, are looking to cut the deficit on Caley Thistle to three points amid a challenge from Hibernian for a top-six spot.
“We have to try and close the gap on Inverness and Dundee United and we have got to play them both in the next few weeks,” Wright said.
“They will probably think that they can get a win that would almost guarantee them top six, so it’s an important game for us.”