The 26-year-old Belgium international only joined the French side in July for around £15m from Nottingham Forest, which was completed before the profit and sustainability deadline of 30 June.
Mangala, who played at Euro 2024, made 12 appearances on loan at Lyon last season before signing a permanent deal with the French club.
“Everton is a big club,” he told evertontv. “When I’ve played against Everton in the past, I’ve always felt there is a big fanbase here with a lot of passion for their club so it was an easy decision for me to come here.
“I think I’m someone who is composed on the ball. I like to link up play in the midfield between the defensive players and the attack – hopefully a bit of a balance of everything.
“I want to help make it a special last season at Goodison and to help the club reach our goals. Up the Toffees!”
Mangala joined Nottingham Forest from Stuttgart in July 2022 and made 53 appearances for the club, scoring two goals.
This is the Toffees’ seventh summer arrival after the return of winger Jack Harrison on loan, midfielder Tim Iroegbunam, forwards Iliman Ndiaye and Jesper Lindstrom, defender Jake O’Brien and goalkeeper Asmir Begovic.
“We want competition for places throughout our squad and his loan signing gives us extra options and adds depth to our midfield,” added Everton boss Sean Dyche.
“Orel has Premier League experience, which is important to help him adjust as quickly as possible, and his performances for Belgium and in Europe underline the quality he can bring to us at Everton.