She began her career portraying Liberty Ciccone on the CBS soap opera As the World Turns (2008–2010), which earned her a Daytime Emmy Award nomination for Outstanding Younger Actress in a Drama Series in 2009.

Hagner gained further prominence for her role as Portia Davenport in the TBS and HBO Max dark comedy series Search Party (2016–2022), receiving critical praise for her performance.

Hagner's film roles include Hits (2014), Folk Hero & Funny Guy (2016), The Oath (2018), Brightburn (2019), Palm Springs (2020), and You're Cordially Invited (2025).

[7] Hagner also appeared in a recurring role in the medical comedy Royal Pains on USA Network as Libby, a teenager with an extreme case of cyberchondria from 2009 to 2011, and guest-starred on In Plain Sight, CSI: Miami, and The Following.

Hagner began a relationship with actor Wyatt Russell after meeting on the set of the film Folk Hero & Funny Guy in 2015.