She is mostly known for playing alongside Daniel Craig and Christoph Waltz in Spectre.

In 2011, Millar was awarded Best Supporting Actress for her role in the British feature film David is Dying.

In 2014, Millar was one of the supporting roles of The Quiet Hour with Dakota Blue Richards, which was nominated for Best UK Feature at the 22nd Raindance Film Festival.

Millar portrayed SPECTRE's Dr. Vogel alongside Daniel Craig and Christoph Waltz in Spectre the following year.

In 2018, Millar played alongside Matt Passmore in the short film Nox directed by Keyvan Sheikhalishahi.