After the war Leigh-Mallory entered the legal profession but in 1919 he returned to the recently-created Royal Air Force. Promoted to the rank of squadron leader he specialized in Army cooperation. In 1931 Leigh-Mallory became deputy director of staff studies in the Air Ministry and commander of No. 2 Flying School. He also served as a staff officer in Iraq before being appointed commander of 12 Fighter Group in 1937.
During the Battle of Britain Leigh-Mallory quarrelled with Air Vice-Marshal Keith Park, the commander of 11 Group. He wanted his squadrons to be closely involved in action with the Luftwaffe, but Park, who was responsible for the defence of southeast England and London, complained that 12 Group was not doing enough protect the airfields in his area. Instead, Leigh-Mallory and Acting Squadron Leader Douglas Bader had devised a massed fighter formation known as the Big Wing which they used, with mixed success, to hunt German bomber formations.
In return, Leigh-Mallory was critical of the tactics of Park and Sir Hugh Dowding, head of Fighter Command, believing that not enough was being done to allow wing-sized formations to operate successfully. Air Chief Marshal Charles Portal, the new chief of the air staff, agreed and wanted Dowding replaced. After the Battle of Britain was over Portal removed Keith Park and Hugh Dowding from their posts. Leigh-Mallory took over from Park as commander of 11 Group. As a beneficiary of the change in command Leigh-Mallory has, probably unjustly, been accused of forming a plot to overthrow Dowding.
In November 1942, Leigh-Mallory replaced Sholto Douglas as head of Fighter Command. He was knighted in January 1943 and later that year was appointed commander of the Allied Expeditionary Air Forces for the Normandy invasion.
In August 1944 Leigh-Mallory was now appointed Air Commander-in-Chief of South East Asia Command (SEAC). But before he could take up his post he was killed en route to Burma when the aircraft he was travelling in crashed into the French Alps. All on board, including his wife, died. Leigh-Mallory, by then an Air Chief Marshal, was the highest-ranking British officer killed in the World War II.
The subsequent Court of Inquiry found that the accident was a consequence of bad weather and may have been avoided if Leigh-Mallory hadn't insisted the flight fly in such poor conditions. By a quirk of fate, Leigh-Mallory's replacement at SEAC was Air Marshal Sir Keith Park.