The German World Cup winner became a man at Goodison Park.
Everton snapped up the young German from Hamburg in 2009 but he played just one game for the club against BATE Borisov in the Europa League.
By January 2012 Mustafi had decided he wanted regular game time. He told David Moyes he wanted a transfer and he was granted his wish - Everton let him join Italian side Sampdoria on a free transfer.
He has gone on to become one of Europe's most coveted central defenders. He is currently starring for current club Valencia and in the summer of 2014 he helped Germany to a memorable World Cup triumph.
But he insists that his time at Everton was not entirely wasted.
Speaking to FIFA.com this week he stated:
"As a 17-year-old it was difficult to accept that I wasn't featuring. I felt like I had to play more because I was still developing.
"But I became a man in England. I moved there as a youngster and during my two-and-a-half years at Everton I matured and my physique developed.
"It reached a point where I couldn't go on like this. I had to take the next step, I had to play."
It would certainly appear that Mustafi is in some way thankful for Everton. They clearly worked on the physical side of his game and the difficulties he endured no doubt made him a better player in the long haul.
Valencia are now reaping the benefits of Mustafi's school of hard knocks. If only Everton had given him more of a chance.