News that the striker was to sign for the Blues was leaked on Twitter last week but the club have now announced his capture after he put pen to paper on a two-year deal.
He came through Lyon's youth academy before joining Metz two years ago and he made his Ligue 1 debut earlier this year.
“I am very happy and honoured to sign for Everton,” he told evertonfc.com after the formalities of his move were completed. “It is a great club and I have always liked English football and the Premier League.
“I have followed Everton from France. The Club has some great players and I'm very excited to become part of it.
“My dream is to play for the first team here,” he added. “Ronald Koeman is a huge figure, not only as a coach now but his reputation as a player speaks for itself as well.
“I want to play in the Premier League for Everton.”
Mathis will be joined at Finch Farm by Norwegian prospect Einar Iversen who has signed up to Everton's Academy on a two-year contract from Stord.
The 16-year-old attacking midfielder has had extensive trials at Everton's Halewood training facility since April and is viewed as an exciting addition to the U18s group.
"It is of course very exciting. I'm moving on Saturday. I have butterflies in my stomach," he is quoted as saying by TV2 in Norway.
"I'm dreaming about playing football and playing in a league like the Premier League. This is definitely a step in the right direction."