Lily, who plays rebellious seductress Lady Rose in the period drama, displays some impressive horseriding skills in the first photo released of the film.
"With Lily James we have found our perfect Cinderella," said director Kenneth Branagh. "She combines knockout beauty with intelligence, wit, fun and physical grace."
The 24-year-old will undergo the journey from rags to riches alongside Cate Blanchett as her cruel stepmother and Helena Bonham-Carter as her fairy godmother.
The ugly stepsisters will be played by Holliday Grainger and Lily's fellow Downton Abbey actress Sophie McShera.
The Prince Charming poised to put the sparkling slipper on Cinderella's foot is Game of Thrones star Richard Madden.
Filming has begun at Pinewood Studios in London and will continue in various locations around England.
The live action adaptation of the timeless tale will be released in 2015.