The Englishman is fondly remembered around Gorgie for his role in the thumping 5-1 win over rivals Hibernian in the 2012 Scottish Cup final.
Grainger netted the third goal from the penalty spot just after half-time at Hampden Park after Pa Kujabi had pulled Suso Santana to the ground.
Just 32 years of age, the left-back has decided to call it a day on his playing career “unless Manchester United come in with a £300k-a-week offer”.
Grainger has spent the last five years with boyhood club Carlisle United and feels this is the right time to call it quits with the player picking up more niggly injuries, which has meant he has played just 50 per cent of the minutes in League Two this campaign.
He told United’s official website: “I’ve always said that I wanted to finish my career here. Being on year-to-year contracts, and knowing I only had a year left, it was one where you always go into each season thinking it could be your last one.
“It hasn’t been a rash decision, or anything like that, it’s been something that’s been coming. I just feel that, moving forward, the club needs some fresh blood and a fresh start. The manager has sort of said himself that there’ll be changes in the summer, so it felt like the right time for me to step away from the club.
“The knock-on effect is that me finishing at this club is me finishing my career.”
He added: “It’s been a massive decision to make. I’ve spoken to many people, team mates past and present, friends and family. It’s been a tough decision. My wife Heather has been great, and she’s asked me 100 times if I’m sure it’s the right decision, but we both think it is.
“I’ve spoken to my parents as well and they both think it’s the right time. It hasn’t been an easy decision, but we all feel like it’s the right time to step away.”
Grainger made 55 appearances for the Jam Tarts, scoring four goals during his two years at the club.
Hearts were one of six clubs he played for in Scotland but Hearts brought the fondest memories.
“I love it up there,” he said. “The massive grounds, the big clubs, the derbies. Everything was a learning curve for me.
“(Hearts) was one where I made a lot of good friends there. We went through a hard time not getting paid, things like that. It was the club I probably grew closest to when I was up there. I go back up whenever I can.
“To be able to play in the cup final was unbelievable, to score against your biggest rivals, they are memories you will never lose.”
He is regarded as ‘Mr Carlisle’, scoring the club’s 5,000 Football League goal in 2017.
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