Benedict Cumberbatch, Colin Firth and Mark Strong have joined the production, along with The Bodyguard star Richard Madden. They will join the previously announced stars George MacKay and Dean-Charles Chapman, who will play young British soldiers on a single day at the height of World War I.
Andrew Scott, Daniel Mays, Adrian Scarborough, Jamie Parker, Nabhaan Rizwan and Claire Duburcq have also joined the cast.
Mendes will direct from a screenplay he wrote with Krysty Wilson-Cairns.
Mendes will also produce the film via his Neal Street Productions with Pippa Harris, along with Jayne-Ann Tenggren and Callum McDougall. Co-producing is Michael Lerman.
Along with Neal Street, DreamWorks Pictures and Universal Pictures will produce.
The film will begin principal photography next week, shooting on location in England and Scotland.
Universal will release the film domestically on December 25, 2019.