He was born in Tromsø as the son of carpenter Ingbert Kristiansen (1881–1968) and Ingeborg Haugen (1886–1976). The family moved to Trøndelag when Kristian was nine years old. After middle school he held several different jobs. Starting in 1936, he published a large number of short stories in the magazine Arbeidermagasinet. In the late 1930s he published two short plays, Det dages (1937) and Medaljen (1938), and the children's book Eggtjuver i fugleberget (Egg thieves at the nesting cliff, 1938). His first novel, Vi bærer et bilde, came in 1939. His youth's book Jeg er ingen spion! (I am no spy!) from 1940 is one of the very few books published in Norway throughout the Second World War where it is told from the unoccupied parts of the country during the battles in 1940.
During the occupation of Norway by Nazi Germany, which started in 1940, Kristian became involved in resistance work. In 1942 he had to flee to Sweden. There he met the physician Ellen Hedlund; their subsequent marriage lasted from 1943 until 1955. In 1959 he married architect Turid Ellefsen.
Kristiansen had moved to Trondheim in 1950, and his novels published after 1950 were historical novels from Trondheim. The trilogy Adrian posepilt (1950), Vårherres blindebukk (1952) and I den sorte gryte (1954), about a boy growing up in an orphanage in the late 17th century, is regarded as Kristiansen's main work, and made him known throughout the country. The novel Klokken på Kalvskinnet (1966) is about the sufferings of a labour prisoner from the early 19th century.