The newly-arrived Mustang was quickly recognized as being the best fighter aircraft yet to be delivered from the USA. It was found to be superior to the Kittyhawk, Airacobra and Spitfire in both speed and maneuverability at low altitudes. Maximum speed was 382 mph at 13,000 feet. At all heights up to 20,000 feet, the Mustang was faster than any other fighter then in service with the RAF. Rate of climb, acceleration, speed in a dive, stability, handling in all configurations, rate of roll and radius of turn were all rated as being satisfactory to outstanding. The armament of four 0.50-inch and four 0.30-inch machine guns of the Mustang I and the four 20-mm cannon of the Mustang IA was heavy and effective. The range was nearly double that of any RAF single-engined fighter. It was 25 to 45 mph faster than the Spitfire V at altitudes up to 15,000 feet. The problem was the rapid fall-off in performance at altitudes above 15,000 feet, caused by its low-altitude Allison engine which was supercharged for best performance at low levels. The Spitfire could climb to 20,000 feet in seven minutes, while the Mustang required 11. Both the Spitfire and the Messerschmitt Bf 109 were more nimble at higher altitudes. The Mustang weighed about a third again as much as a Spitfire, and was considered as being somewhat underpowered.
The relatively poor high altitude performance of the Mustang was more than just a minor deficiency, since most aerial combat over Europe at that time was taking place at medium to high altitudes. Consequently, it was decided that the Mustang I could be best used for low-level tactical reconnaissance and ground attack, where full advantage could be taken of its exceptional low-altitude performance.
The first RAF unit to receive the Mustang was No 26 Squadron at Gatwick which began to operate the fighter in February 1942. In April, two more squadrons received Mustangs, and eight more in June. Most of the aircraft went to Army Cooperation Command, usually replacing Curtiss Tomahawks or Westland Lysanders. The first Mustang combat mission was undertaken by Flying Officer G. N. Dawson of No. 26 Squadron on May 10, 1942, strafing hangars in France and shooting up a train.
It was initially feared that the Mustang I might be mistaken for a Bf 109 during the stress of combat, and most of the Mustang Is in front-line RAF service had bright yellow bands painted across their wings.
The first Mustang I operational sortie was on July 27, 1942. Mustang Is participated in the disastrous Dieppe landings by British commandos on August 19, 1942, where it saw the first air-to-air action. During this operation, pilots of No 414 Squadron of the RCAF were attacked by Fw 190s. An American RCAF volunteer, F/O Hollis H. Hills, shot down one of the enemy, which was first blood for the Mustang.
In October of 1942, On a mission to the Dortmund-Elms Canal and other objectives in Holland, the Mustang I became the first single- engined fighter based in the UK to penetrate the German border. By this time, the Mustang I equipped Nos 2, 4, 16, 26, 63, 169,239, 241, 268, and 613 Squadrons of the RAF, plus Nos 400, 414 and 430 Squadrons of the RCAF, and No 309 (Polish) Squadron of the RAF.
Tactical reports from RAF army cooperation units were laudatory. The Mustang I and IAs were able to take an incredible amount of battle damage. The long range of the Mustang made it an excellent tactical reconnaissance aircraft and its heavy armament made it effective against most ground targets. In 18 months of operation 200 locomotives and 200 barges were destroyed or severely damaged, and an undetermined number of enemy aircraft were destroyed on the ground. This was accomplished at the expense of only one Mustang being shot down by enemy fighters, five lost to flak, and two vanishing with no record of their fate. At low altitudes, the Mustang was faster than either the Bf 109 or the Fw 190. At sea level, the Mustang could run away from any enemy aircraft. The flaps were very useful in combat to reduce the turning radius.
Mustang Is and IAs served with the RAF up until 1944. It knew few equals in the role of low-altitude interdiction and reconnaissance.