Fjærli was born in Molde, the son of the politician Olav Eysteinson Fjærli. When Germany invaded Norway in 1940, he was studying philology, and as a conscript sergeant he participated in fighting in Eastern Norway. In the fall of 1940 he went on an intelligence mission to the United Kingdom, where he joined the Norwegian Brigade in Scotland. In the fall of 1943, Fjærli became a liaison officer in the British 52nd Mountain Division, which fought in the Netherlands. On May 9, 1945 Fjærli returned to Norway as the chief liaison officer for the Allied forces in Southern Norway.