He has been featured in the films Into the Woods (2014), Bridge of Spies (2015), Birth of the Dragon (2016), Game Night (2018), and Aladdin (2019), and has had supporting television roles in Get Shorty (2017) and Maniac (2018).

In 2021, Magnussen starred in the sci-fi series Made for Love on HBO Max and appeared in the films The Many Saints of Newark and No Time to Die.

He has starred in Broadway and off-Broadway theater productions, including 2013's Vanya and Sonia and Masha and Spike, for which he received a nomination for the Tony Award for Best Featured Actor in a Play.

[6] In 2012, he was cast alongside David Hyde Pierce and Sigourney Weaver in Christopher Durang's Vanya and Sonia and Masha and Spike, premiering at the McCarter Theatre in Princeton, New Jersey, prior to its move to Lincoln Center in New York City.

[7] Magnussen appeared in the films Blood Night: The Legend of Mary Hatchet and Happy Tears, which stars Demi Moore and Parker Posey.

He played Neil Thomas, a World War II hero, in the 2011 film The Lost Valentine, based on the book of the same name by James Michael Pratt, which aired on CBS on January 30, 2011.

In 2019, Magnussen played the newly created role of Prince Anders in Disney's live action adaptation of Aladdin, directed by Guy Ritchie.