A deal for Hazard was finalized on Friday after months of speculation.
Mendy is expected to be announced as a Madrid player following commitments with the French national team. He reportedly completed the first phase of his medical at Clairefontaine, France's training base, this week.
Mendy, 23, is part of Didier Deschamps' squad for the upcoming Euro 2020 qualifiers against Turkey and Andorra.
Known for his enterprising play on the left flank, Mendy is considered a long-term replacement for Marcelo. The Brazilian sat out chunks of last season as then-manager Santiago Solari favored 22-year-old Sergio Reguilon.
Zinedine Zidane restored Marcelo to the starting lineup after succeeding Solari in March, but it appears the Frenchman is now looking to the future.
Los Blancos already signed Jovic from Eintracht Frankfurt for an estimated €60 million and Militao from Porto for €50 million. Hazard reportedly cost €100 million.