The Premier League giants decided to sell the defender to the German club this week for a transfer fee of €2m, ending his time at Old Trafford.
Fosu-Mensah has been a bit-part player in the Manchester United squad since last season and was looking for a fresh start away from the club.
Leverkusen snapped him up on a permanent deal and the Dutchman will try to reinvigorate his career in Germany over the next few seasons.
There have been suggestions Manchester United have reserved an option to sign him from Leverkusen, but according to German outlet Fussball Transfers, no such arrangement is in place.
There is no such clause in the agreement between the two clubs that would allow Manchester United to buy him back in the future.
Fosu-Mensah joined Manchester United from the Ajax academy in 2014 and made 30 senior appearances for the club.
He has signed a three-and-a-half-year contract with Leverkusen and has managed to get a clean break from Manchester United.
The Dutchman also has three caps for the Netherlands and will look to force his way back into the national team with his performances in Germany.
Manchester United could try to re-sign him in future, but have no clause which would make the process easy.
