The Spanish side released a statement yesterday saying they had reached an initial agreement for the French defender to join the club until June 2021.
Mangala joined Manchester City for €34.5million from Portuguese side Porto in 2014, but has not played since February last year due to a knee injury.
During his time in Manchester, he was signed by Valencia on loan, where he made 33 appearances.
The 28-year-old has also played 8 times for France, with his last call-up being in 2016.
Valencia begin their La Liga season at home to Real Sociedad on Saturday although Mangala may need more time to regain match fitness.