Since 2018, she has played Beth Dutton on the Paramount Network neo-Western drama series Yellowstone, opposite Kevin Costner.

The following year, she appeared in an episode of the Carlton UK television period drama series Bramwell playing a disturbed young woman.

[10] In 2002, Reilly starred alongside Audrey Tautou and Romain Duris as Wendy, an English Erasmus student, in the French comedy L'Auberge espagnole (The Spanish Apartment).

She reprised her role in the 2005 sequel, Les Poupées russes (Russian Dolls) and the 2013 follow-up, Casse-tête chinois (Chinese Puzzle).

The same year, Reilly made her Broadway debut opposite Clive Owen and Eve Best in Harold Pinter's play Old Times at the American Airlines Theatre.

In 2017, Reilly played the Celtic Queen Kerra, confronting the Roman invasion of Britain in Sky TV's Britannia.

She was cast in the leading female role in the Western-style U.S. series Yellowstone, a Paramount Network drama that debuted on 20 June 2018.

[13] Reilly's performance in After Miss Julie at the Donmar Warehouse made her a star of the London stage and earned her a nomination for a Laurence Olivier Theatre Award for Best Actress of 2003.

[8] In 2005, she won Best Newcomer Award at the Cannes Film Festival for her role as Wendy in Russian Dolls (Les Poupées Russes).

[8] In 2006, Reilly won the Empire Award for Best Newcomer for her role in the British comedy film, Mrs Henderson Presents.