His interest in aviation came early. He watched training aircraft fly over the family farm on their way north to Canada, and the sight stayed with him. He enrolled briefly at Utah State University, then found his way into the Civilian Pilot Training Program, a federal initiative that trained pilots in the years before American entry into World War II. He earned his private pilot license through the program in September 1941.
After the Japanese attack on Pearl Harbor in December 1941, Halvorsen joined the Civil Air Patrol and then enlisted in the U.S. Army Air Forces in May 1942. He trained at the No. 3 British Flying Training School in Miami, Oklahoma, run by the Spartan School of Aeronautics. He had expected to fly fighters, but the Army Air Forces needed transport pilots and he was reassigned. He spent the war years flying transport operations in the South Atlantic Theater, gaining experience on large multi-engine aircraft that would define the rest of his career.
By 1948 he was based in Mobile, Alabama, flying C-74 Globemasters and C-54 Skymasters. The Soviet blockade of Berlin began in June 1948, and by July 10, 1948, Halvorsen had his orders assigning him to Germany and the airlift. He was twenty-seven years old and a competent, experienced transport pilot. His initial orders called for roughly twenty-three days of service. He ended up staying for about seven months.
He flew into Tempelhof on aircraft loaded with flour, dried eggs, dried milk, and coal. His first Berlin cargo was approximately 20,000 pounds of flour. The city he landed in was tense, busy, and unlike anything he had seen before: a former enemy capital now dependent on American transport pilots for its daily bread.