Christie took his examen artium in 1935, and attended the Technische College of Berlin from 1935 to 1937 and "Hærens Flyveskole" from 1937 to 1939. As World War II broke out, Christie, with the rank of sergeant, was called to Sola Airport as a member of Norway's neutrality guard. However the airport was attacked by Germany on 9 April 1940 and fighting ensued. The Norwegians lost the battle, but Christie managed to retreat to the inner country. His group was involved in fighting until 23 April, whereafter it retreated further east. Norway eventually fell to German rule, and in the autumn of 1940 Christie fled to Canada via Sweden, the Soviet Union and the United States. He joined the air force-in-exile at Little Norway, and spent the rest of the war as a fighter pilot. He commanded the Norwegian No. 332 Squadron, then the RAF's 234 Squadron, and as wing leader of 150 Wing and the Hunsdon Wing flying the P-51 Mustang, reaching the rank of lieutenant colonel. After 244 operational missions he was shot down over Germany on 18 April 1945 and held as a prisoner-of-war until the war's end. Not many from his "Hærens Flyveskole" class survived the war.