With Douglas Luiz having been sold to Juventus before the end of June to help Villa comply with the Premier League's Profit and Sustainability regulations, a replacement was a top priority of Unai Emery.
Reports emerged earlier this month that acquiring the services of Onana had become Unai Emery's preference, regardless of a deal costing in the region of £50m.
The West Midlands outfit have now revealed that they have finalised the transfer of the Belgium international on a long-term contract.
Onana departs Everton after two years at Goodison Park, the Toffees making a considerable profit on the alleged £33m that they paid in 2022.
Onana reveals Tielemans influence
Despite arguably not living up to his potential with Everton, Onana has just been a mainstay for Belgium at Euro 2024, where he made four starts.
During that time, the 22-year-old featured alongside Villa midfielder Youri Tielemans in the Red Devils engine room, and he has acknowledged that it played a part in his decision.
Speaking to the club's official website, Onana said: "Youri played a big part in me being here because he told me lots of good stuff about the club.
"He's someone I'm used to playing with in the national team, so I'm looking forward to training with him on a daily basis.
"He's someone who's got the experience to help and guide me throughout the whole process."
Onana also added: "There's a healthy atmosphere here to grow as a player and as a man.
"We also have a manager that's one of the best in the business and I think he can take my game to the next level."
Yet another deal between Villa, Everton
These two clubs have been in contact for much of the summer with Villa midfielder Tim Iroegbunam having made the move to Merseyside.
Versatile Villa forward Lewis Dobbin went in the opposition direction, both deals helping the teams comply with the financial rules.
Everton will now enter the market for a replacement for Onana, someome which will almost certainly cost considerably less than the fee that they have recouped as they look to strengthen several areas of the pitch.