According to Variety, the British helmer will work on adaptations of the novels Butcher's Crossing and Middlemarch.
Butcher's, penned by John Williams in 1960, is about a man who heads to Kansas to herd buffalo. The novel has drawn critical praise for its deconstruction of the Western genre.
George Ellot's novel Middlemarch will also be developed by scriptwriter Andrew Davies (Brideshead Revisited, Little Dorrit) with a view to Mendes directing. Set in the 1830s, it is made up of several interlocking stories exploring changes in a fictional Midlands town.
"The range of Sam's interests and skills as a filmmaker is limitless, and he is a magnet for top talent in front of and behind the camera," said Focus CEO James Schamus of the deal.