She started her acting career in Julie Taymor's 1999 London production of The Lion King, as one of the original young Nalas.

In 2006, she got a place at Royal Academy of Dramatic Art and also the role of Emmie Thibodeaux in the musical Caroline, or Change, for which she was nominated for the Whatsonstage.com Stuart Phillips London Newcomer of the Year award 2007, alongside Andrew Garfield. She went on to star in the lead role in Athol Fugard's UK premiere of Victory for the Peter Hall Company and then went to RADA in September 2007.

Bennett-Warner graduated from RADA in 2010, but left early to take on the role of Sophie in Lynn Nottage's Ruined at the Almeida Theatre. Before completing the course she was awarded the prestigious Carleton Hobbs Radio Award. However, due to another job commitment she was unable to join the radio rep.

In 2012 the Radio Times named Bennett-Warner as one of their Rising Stars.

In 2013 she joined the main cast of Sky 1's The Smoke alongside Jamie Bamber and Jodie Whittaker.

In 2014 she played the female lead opposite Ashley Walters in an episode of Dominic Savage's TV mini series The Secrets. She followed this by playing Saibra in Doctor Who opposite Peter Capaldi, Jenna Coleman and Jonathan Bailey. She also played Mary Thorne in the BBC radio 4 production of Dr Thorne alongside Iain Glen and Douglas Booth.

In 2015 she narrated Helana and the Beast an original Doctor Who audiobook.

In 2016, she narrated Zadie Smith's book Swing Time.

In 2017, she co-starred in The Foreigner, with Jackie Chan, Sick Note, alongside Rupert Grint, Nick Frost and Don Johnson, and Harlots, with Samantha Morton and Lesley Manville.