In 2008, Montgomery was cast as the love interest of the character Tony Stonem in an episode of the teenage drama Skins. She also appeared in both the TV movie Dis/Connected and the short film Flushed.
In 2010, Montgomery appeared in her first theatrically released films, playing Ruth in the mystery The Rapture, Giselle in the independent horror film Dead Cert and the supporting role of Madeline in the Academy Award nominated psychological thriller Black Swan – playing a Little Swan to Natalie Portman's Oscar-winning portrayal of the Black Swan.
In December 2011, Montgomery starred as Princess Mithian, a love interest of King Arthur, in Series 4 Episode 11 of Merlin, later returning in Series 5 Episode 4 (alongside James Fox).
In 2012, Montgomery played the lead role in the CBS television series Made in Jersey. The series was cancelled after two episodes aired. Despite the early cancellation, Montgomery received huge compliments and a career boost from the short lived series. Two new episodes aired on 22 December 2012.
Montgomery attracted the attention of Stephen Poliakoff via an audition tape she sent to casting Director Andy Pryor for the role of Sarah in Dancing on the Edge. Poliakoff was impressed and offered her the job via Skype. The five-part miniseries aired 4 February 2013 on BBC Two.
In December 2014, The Hollywood Reporter announced Montgomery was cast to star as a prostitute in the comedy Amateur Night alongside Jason Biggs.
In October 2016, Montgomery started recurring in NBC's This Is Us.
