Danish motion-picture director known for his exploration of the macabre.

Christensen began his career as an opera singer in 1902 but later became an actor and then a director. By 1913 he was known as the writer, star, and director of a film exploring the unknown, Det Hemmeligheds fulde X (The Mysterious X), his first investigation of the horror of the macabre. In Sweden between 1919 and 1922 he directed the film Häxan (Witchcraft Through the Ages), for which he became famous. In the film he portrayed Satan, the central character in a screenplay that gave a graphic description of the continuum of satanic practices from medieval to modern times. The film, although widely acknowledged for its craftsmanship and the source of his acclaim as a director, was banned in many countries for its scenes of cruelty and sadism. As a result of his growing reputation in the cinema, he was invited to work in Berlin in 1924, directing three films and acting the lead in Carl Dreyer's Mikaël (1924; Chained).

The years 1926–29 were spent in Hollywood, directing such comedy-mysteries as The Haunted House (1928) and The House of Horrors (1929) with Chester Conklin. Before World War II he returned to Denmark, where he directed three films about social problems, as well as a thriller. He was the manager of a Copenhagen motion-picture theatre for the last 17 years of his life.