Pietrzak joined the Polish Air Force in 1933, and during the Invasion of Poland, piloted PZL.23 Karaś light bombers and later flew fighters with the Free French Air Force's GC III/9 squadron. He joined No. 306 Polish Fighter Squadron as a Sergeant Pilot in August 1941, flying Hawker Hurricanes and Supermarine Spitfires, and was commissioned the following year, eventually becoming a Squadron Leader. On 31 December 1942 while flying a Spitfire Mk IX (Serial No. EN128) he scored the 500th victory for the UK based Polish Air Force in the war and was later decorated by Polish President Władysław Raczkiewicz. After starting a second tour with 306 Sqn he joined 315 Polish Fighter Squadron from July 1944 until October 1944, flying Mustang IIIs. He was awarded the DFC in August 1944.