Sumner started her career in London at age 17. She made her official runway debut with Hedi Slimane at Yves Saint Laurent. She has walked the runway for Sonia Rykiel, Miu Miu, Kenzo, Haider Ackermann, Lanvin, Vionnet, Marc Jacobs, Kate Spade, Diane von Fürstenberg, and Céline.

Sumner has been on the cover of Numéro, Vogue Ukraine, Vogue Czechoslovakia, Vogue Italia, Modern Matter, Double, Purple, Zoo, Document, 10 Magazine, Arena Homme + and more.

Sumner wrote and directed a short film in 2019 titled Lucan Asks Why starring Coco Gordon-Moore and Lilac Cianciolo. The film was part-funded by Gucci and debuted at the Gucci Wooster Store in New York City.

In 2022 Sumner debuted short film Angel Song, a collaboration with Maryam Nassir Zadeh, at The Roxy Cinema in New York City.