DreamWorks Pictures and Universal Pictures announced the casting news Thursday.
Principal photography is to begin on April 1 and the movie is set for theatrical release on Dec. 25.
The film chronicles a single day in the lives of two young British soldiers played by George MacKay from Captain Fantastic and Game of Thrones alum Dean-Charles Chapman.
Rounding out the cast are Daniel Mays, Adrian Scarborough, Jamie Parker, Nabhaan Rizwan and Claire Duburcq.
Mendes has previously helmed films Skyfall, Road to Perdition and American Beauty.