The 46-year-old "Bloodline" actor will join Olivia Cooke, who is already on board, said The Hollywood Reporter.
The Warner Bros. And Village Roadshow project that adapts the best-selling sci-fi novel by Ernie Cline, is still looking for the male lead.
"Ready Player One" is set in a virtual world called Oasis, in which a teenager finds himself competing in a treasure hunt against ruthless foes after the game's founder dies and offers his fortune as the grand prize.
Mendelsohn would play an executive for an Internet corporation that has designs on Oasis.