After a good season for the SønderjyskE under-17 squad, Christiansen signed a three-year youth contract with the club.[1] He made his senior debut on 8 December 2018, against Hobro IK, where he started on the pitch but was replaced by Rasmus Vinderslev in the 68th minute.[4] Later that month he signed a new three-year contract, starting after the expiry of his youth contract in summer 2019.[5]
On 17 June 2021 Christiansen signed on loan for FC Helsingør for the 2021–22 season.[6] The loan ended early in December 2021, due to lack of playing time at Helsingør.[7]