Born and raised in the town of Slagelse in the western part of Zealand in Denmark, Andersen discovered his love of acting while studying drama at school. He participated in many musical oriented plays and was fortunate to gain a lot of experience with many big roles. Like his peers, he began auditioning for film roles.
At age seventeen, Andersen realized the difference between live performances and acting in front of a camera. This encouraged him to pursue Film and Media Studies at the University of Copenhagen. He has appeared in numerous television and film projects in Denmark. Vikings marks his Hollywood debut.
When he is not acting, Andersen enjoys spending much of his time focusing on photography. Considering himself a visually-minded person, he continues to learn more and more about cameras due to his experience shooting and directing short films. In addition, he has learned dancing, singing, sports, fencing, and stunts. He gives back to charities such as UNICEF, Care Denmark, and Kræftensbekæmpelse (Fighting Cancer).