The 17-year-old agreed the move from Hamburg in May and the formalities have now been completed over a three-year deal.
"It's a very nice feeling," Mustafi told evertonTV.
"I had three good years at Hamburg but now this is a new chapter. It was a hard decision to leave home but it's another country, another language and I'm looking forward to it.
"I hope I have a good start and I can learn to improve on my weaknesses. Then I hope I can move through the reserves into the first team and get a chance to prove myself. I feel like I will get that chance here."