Munro, who turned 33 last month, has started six games this season but decided to plan for the future after realising he would be understudy to Brian McLean and Scott Boyd in central defence.
The defender joined County in the summer of 2011 after being released by local rivals and home town club Inverness, where he made more than 350 appearances.
Munro told Moray Firth Radio: "When you turn 30, your mind looks to the end of your career and the next stage. We have another kid on the way as well, to make it three, so that enhances it.
"I asked the manager if my time was up and we both came to the agreement. He didn't want to stand in my way if that's how I felt, so credit to the manager and (director of football) George (Adams) for getting things done so quickly.
"Everyone knows when it's time for change. I wasn't going to play as much as I wanted to.
"I need to go into the world now and the next part of my career.
"I wasn't going to go beyond the summer anyway so it gives me a few months' head start to go into a new career."
Manager Derek Adams told the club's official website: "Grant had a terrific two years with the club, winning the First Division Championship and finishing fifth in the SPL last season. He goes with our club's best wishes."