He becomes their second signing of the summer transfer window, joining goalkeeper Maarten Stekelenburg in moving to Goodison Park.
He has signed a four-year deal at the club and will wear squad number 17 as Ronald Koeman steps up his transfer business ahead of the new season.
The Blues made their move for the Senegal international last week, activating a £7.1 million release clause in his contract.
They had to wait until this week to complete the transfer, with the midfielder completing his medical at the club’s Finch Farm training complex on Tuesday.
There had been some competition for his signature, with West Ham and Marseille both reportedly making late bids but he has elected to make Merseyside his new home.
Koeman will be keen to see him replicate the form he showed despite Villa’s relegation from the Premier League last season.
Only Leicester City star N’Golo Kante bettered him in a number of statistical areas last season, and the Dutchman will be hoping he can produce similar impressive stats in the Toffees midfield next season.