Sumner was born in London, the oldest daughter of musician Sting (born Gordon Matthew Thomas Sumner) and Actress Trudie Styler. Her siblings include Jake Sumner, Eliot Sumner, and Giacomo Sumner. She also has two older half-siblings, musician Joseph Sumner (born 1976) and Actress Fuschia Katherine "Kate" Sumner (born 1982), both from her father's previous marriage to Actress Frances Tomelty.
Sumner began her acting career in 2006 with roles in a series of short films. In 2011, she landed the role of Francesca on the Showtime series The Borgias appearing in four episodes. Sumner garnered significant attention for her role as Sophie Levee, starring opposite Greta Gerwig, in 2012's Frances Ha.
Sumner was featured as Katia on the AMC Network series Low Winter Sun. In 2013, Sumner acted in a series of films including Half the Perfect World, The Mend and Anesthesia.
Sumner recently starred in the independent film The Mend, alongside Josh Lucas and Stephen Plunkett. The film premiered at the SXSW Film Festival in March 2014.
She became engaged to Chris Kantrowitz in June 2016, and they married in Tuscany in July 2017.
