Mary Duff Stirling Smurthwaite, Lady Twysden (22 May 1891 – 27 June 1938)[1] was a British socialite best known for being the model for Brett Ashley in Ernest Hemingway's 1926 novel The Sun Also Rises.

[2] She was the eldest child of Baynes Wright Smurthwaite by his wife Charlotte Lilias Stirling.

[3] On 4 January 1914 her engagement to John Churchill Craigie, son of Pearl Richards Craigie, was announced,[4] but her first marriage was to Edward Luttrell Grimston Byrom, son of Edward Byrom DL of Culver, Devon and Kersal Cell, Lancashire, who served as High Sheriff of Devon in 1888, by his wife Florence Maria, daughter and co-heiress of Marmaduke Jerard Grimston, of Grimston Garth and Kilnwick.

[6] Her second marriage was at Edinburgh on 26 January 1917, to Sir Roger Thomas Twysden, a naval officer.

In the 1988 miniseries Hemingway, starring Stacy Keach, Duff Twysden was played by Fiona Fullerton.