In July 2000, after being accepted into the Academy of Theatre, he was interviewed by the Aftenbladet. He described himself as being from Stavanger in south-western Norway, and spoke in a Stavanger dialect. He had studied drama in upper secondary school and auditioned unsuccessfully for the Academy of Theatre a year earlier. He tried again in 2000, and was invited to try for a third time, in which he succeeded.

After Bikubesong, he played many parts at the same theater, including leading roles in Herr Bima og herr Bramati by Tord Akerbæk (2003) and Skråninga by Carl Frode Tiller (2004).

Hagen made his film debut in Cry in the Woods (Den som frykter ulven) in 2004. He appeared in several other films, including Erik Poppe's 2008 film Troubled Water (De usynlige). For this film he won the Kanonprisen for best performance by an actor in a leading role. A 2012 article in Filmmagasinet praising Troubled Water as a great Norwegian film gave part of the credit for its excellence to “unbelievably good acting” by Valheim Hagen and other members of the cast.

In 2006 he became an official member of the repertory company at Det Norske Teatret, and performed there in Verdas mest forelska par (World's Most-in-Love Couple), Få meg på, for faen! (Touch Me, For God's Sake!), and Gabriel García Márquez's Ingen skriv til obersten (Nobody Write to the Colonel).

In 2008, in addition to Troubled Water, Valheim Hagen appeared in four other well-received films: Cold Lunch (Lønsj), House of Fools (De gales hus), Max Manus, and The Storm in My Heart (Jernanger).

A January 2009 profile in Aftenposten noted that he was in three different movies that were running simultaneously in Oslo cinemas – The Storm in My Heart, Max Manus, and Troubled Water – and was also rehearsing the “surrealistic comedy” Eit lykkeleg sjølvmord at Det Norske Teatret.

In the autumn of 2010 Valheim Hagen played Edmund Tyrone in the Riksteatret's production of Eugene O'Neill's Lang dags ferd mot natt (Long Day's Journey into Night). Liv Ullmann played his mother, Mary Tyrone, and Sundquist played his father, James Tyrone. In an article about this production, a Writer for Dagbladet noted that Sundquist had played Falstaff to Valheim Hagen's Prince Hal in 2008, and that they had played father and son the year after in the film Jernanger, “in roles that were too small for their talents.”

In the autumn of 2011 Hagen played the lead role in a four-part, 4-hour Norwegian TV mini-series, Buzz Aldrin, What Happened to You in All the Confusion? (Buzz Aldrin, hvor ble det av deg i alt mylderet?), based on the debut novel by Johan Harstad.

In the December 2012 issue of the Norwegian edition of fashion magazine Elle, Hagen was named Norway's 14th sexiest man.

In 2014, he played Stavrogin in Demoner 2014, based on Fyodor Dostojevskij's The Devils. The play premiered on February 8th at the National Theatre in Oslo.

In early 2017 Hagen starred alongside Sven Nordin in the thriller series Valkyrien. In summer 2017 it was screened in the UK on Channel 4.