Joining Edinburgh from Ulster in April 2005, the universally popular forward has played a vital role - both on and off the pitch - in Edinburgh's development.
Recording his 50th appearance for the club against Toulouse 13 months ago, New Zealand-born Mustchin went on to qualify for Scotland on residency grounds and played a heroic role in helping the national team defeat Argentina in Buenos Aires.
Now he is planning to embark on the next chapter in his career as player-coach of the Osaka-based team, NTT Docomo Kansai, but revealed the time he has spent with Edinburgh has been the highlight of his career.
Mustchin said: "I have spent four really enjoyable years at Edinburgh. When I came here some people saw me as just being a journeyman rugby player, but I will leave as a full-capped player.
"It has been awesome and I have loved every minute. If you would have asked me four years ago if I was going to be a Scottish international then I would have laughed in your face.
"My son, Sam was born here, so he will always have a bond with the country and my daughter Cara has grown up here, but we felt it was time for a new challenge."
In terms of the high points during his stay in the country, Mustchin cites Edinburgh's fourth place finish last season and also his performance during Scotland's win in Buenos Aires as the peaks.
He said: "Last season was among the highlights for me when Edinburgh finished fourth in the league. Being so heavily involved in that achievement was a highlight for me - for a 31-year-old to play 27 games in a season wasn't a bad effort.
"Just to be asked to play for Scotland was a highlight, but the second game against Argentina was when I played as well as I ever have and ever will; that was memorable."
Part of Mustchin's remit at NTT Docomo Kansai will be to coach the forwards and he refused to rule out a return to Scotland as a coach at some point in the future.
He said: "A lot of people know that I want to get involved in the coaching side of the game.
"So potentially there may be a chance for me to return to Scotland as a coach at some point in the future.
"If that opportunity did happen then I would certainly look long and hard at it. I know Scotland so well, so I wouldn't have any hesitation at coming back."
Still recovering from ankle surgery, Mustchin says he is determined to play some part in Edinburgh's challenge to surpass last season's fourth place finish and leave on a high.
He said: "At the moment, I just want to get fit again and get my ankle right. There will be a few games left that I could potentially be available for, so I want to put my hand up to the coaches and play.
"I said to Andy (Robinson) that I didn't want to finish my Edinburgh career sitting on a bench at Glasgow looking at my damaged ankle - I want to finish on a high note."
Head coach Andy Robinson thanked Mustchin for his service at the club and wished him well as he takes on a fresh challenge overseas.
Robinson said: "Matt has been a great servant for Edinburgh and his contribution to the team during my time here has been of the highest quality culminating in two excellent performances for Scotland in Argentina.
"The whole of Edinburgh wishes him and his family the very best for their move to Japan."
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