On television, Monaghan starred as Maggie Hart in the first season of the HBO anthology crime drama series True Detective (2014).

She played Eva Geller in Netflix thriller series Messiah (2020),[3] and starred in the miniseries Echoes (2022).

With one semester remaining to complete her journalism degree in 1999, she left for New York to pursue an acting career.

[12] Monaghan's first two credited television appearances, in a supporting role, were as Caroline Busse in episodes of Young Americans, which aired in 2000.

She also performed on Law & Order: Special Victims Unit in "Consent", an episode that was broadcast on January 19, 2001, and she made her big-screen debut that same year in Perfume, playing the part of Henrietta.

[17] In May 2018, it was announced that Monaghan had been cast in the main role of CIA operative Eva Geller in the Netflix thriller Messiah.

[citation needed] Monaghan met Australian graphic artist Peter White at a party in 2000.