Schleinhege joined the Luftwaffe before the war and upon completing his training, in February 1941, served as a flight instructor. It was not until April 1942 that he was transferred as an Unteroffizier to combat duty: to serve with 6./Jagdgeschwader 54 Grünherz, at the time stationed near Leningrad. However, it was four months until he scored his first victory and ten months for his 5th victory south of Lake Ilmen. It was only upon his transfer to the JG 54Geschwaderstab that his combat claims picked up. Flying as wingman to the unit commanders, men like the long-serving Hannes Trautloft, Hubertus von Bonin and Anton Mader, gave him invaluable experience for the difficult months ahead. By the end of the year when they were based in Orsha with Army Group Centre, his score stood at 30 victories. He was awarded the German Cross in Gold on 20 March 1944, and scored his 37th victory on 4 April. After an extended period away from the front with officer training, the newly commissioned Leutnant Schleinhege was transferred to 4./JG 54 in Estonia, and resumed scoring on 16 August, and claiming his 50th on 17 September. On 9 October, now based out of Riga, he shot down two Bell P-39 Airacobras and two Il-2 bombers Finally, at the end of November 1944, he was promoted to Staffelkäpitan of 8./JG 54 and by the end of the year, with his squadron based in Libau supporting the troops in the isolated Kurland pocket, his score had climbed to 81. Schleinhege was awarded the Knight's Cross on 28 January 1945, for 84 victories. He and his pilots continued to fly as best they could with the limited fuel supply, covering the naval evacuation of the pocket. Based firstly from Libau until March, then from Heiligenbeil, near Königsberg, until the end of the war. His last flight was on 8 May 1945 when he squeezed his two mechanics into his single-seater Fw190 and flew to Kiel to surrender to the British troops. Schleinhege survived the war, and in three years flew 484 combat missions, all on the Eastern Front and all with JG 54. He ended with 97 confirmed victories and a number more unconfirmed, including 54 Il-2 and Pe-2 bombers.