Mustafi was released by then Everton boss David Moyes in 2012 after making just one substitute appearance during his time at Goodison Park.
The 22-year-old moved to Italy to get his career back on track and while at Sampdoria, he broke into the Germany national team.
Mustafi was originally left out of Joachim Low's squad, but earned a late reprieve due to an injury to Marco Reus and he was on the bench for Sunday's World Cup final win over Argentina.
It has been a remarkable last two years for Mustafi since leaving Everton, with the player making his competitive debut at the finals in Brazil as Germany went on to win their first major trophy since 1996.
"The time I was at Everton, I had to step on because I was not playing," said Mustafi. "I needed to play so I asked David Moyes to release me. I needed to play to grow up.
"Everton were really professional with me and said it wasn't a problem to give me away because I wasn't playing. They always thought about the players, not themselves.
"That is unusual. They wanted to give the players the best opportunity and, for me, that was to move on and go somewhere where I could play.
"I took a step back to go to Serie B in Italy to make two steps forward and now I am here."
Mustafi admits he is still finding it hard to comprehend becoming a world champion.
"The German language has a lot of words, but I don't know how to describe the feeling I felt in those last minutes," added Mustafi. "For a 22-year-old player, it is not so easy to understand what just happened.
"I played my first competitive games for the national team here at the World Cup after being called up for the tournament at the last moment. Now I am lifting the trophy. It is unbelievable."