Breadner was commissioned in the British Royal Naval Air Service on December 28, 1915. During World War I he served on the Western Front in the No. 3 (Naval) Squadron and was promoted to Flight Lieutenant (RNAS) on 31 December 1916. He was awarded the Distinguished Service Cross on May 23, 1917

3 (Naval) Squadron was posted to RAF Walmer during the winter of 1917/1918 . Breadner was released from the RAF with the rank of Major in March 1919.

With the formation of the Royal Canadian Air Force (RCAF) in 1924, Breadner joined as Squadron Leader. He gained a series of promotions: Controller of Civil Aviation in 1922, Wing Commander on April 1, 1924, Director of the RCAF from February 15, 1928, to April 29, 1932, Group Captain on February 1, 1936, and Air Commodore on August 4, 1938.

He became Chief of Air Staff on May 29, 1940, having been promoted to Air Marshal on November 19, 1941. Breadner was promoted on his retirement to Air Chief Marshal, the first Canadian to hold this rank.