Arntzen began her career as a consultant at the Ministry of Justice and the Police's Legal Issues section. From 1989 to 2003, she was a lawyer at the Office of the Attorney General of Norway. She attained to barrister, a lawyer with access to work with Supreme Court cases, in 1993. She then entered the law firm Kluge as a lawyer and partner. Since 2007 Arntzen has been a judge at Oslo District Court.[2]
During the opening of the 2012 trial against Anders Behring Breivik, Breivik claimed that judge Arntzen was a personal friend of Hanne Harlem, sister of former prime minister Gro Harlem Brundtland, whom Breivik on day four of the trial testified had been a principal target for execution during his shooting spree at Utøya island—and whom he had wanted to decapitate on video, Al Qaeda-style—but to the question from Arntzen whether this constituted a formal assertion of conflict of interest, Breivik's main defence counsel Geir Lippestad, after cursorily conferring with Breivik, replied that it was not.[6][7][8]
^Andersen, Gordon: Sæther, Anne Stine; Sørbø, Krister (photographer) (18 April 2012). "Sparket meddommer: – Jeg følte meg litt som en popstjerne" [Dismissed lay judge: – I felt a little bit like a pop star]. Verdens Gang (in Norwegian). Retrieved 20 April 2012.{{cite news}}: CS1 maint: multiple names: authors list (link)