Walter Loos was born on 11 April 1923 at Oppenheim in Rheinhessen. Following flying training he was sent to III./JG 3 in January 1944. He claimed his first aerial victory in the big aerial battle over Berlin on 6 March 1944 claiming a USAAF B-17 Herausschuss. Later he was transferred to IV. Sturmgruppe/JG 3. When Oberst Walter Dahl (129 victories, RK-EL) became Kommodore of JG 300 in July 1944, Loos joined with him to serve on the Geschwaderstab. On 29 September, Loos was awarded the Deutsches Kreuz in Gold. Loos was posted to undertake instructing duties with Ergänzungs-Jagdgruppe Ost in October 1944. He returned to combat duty with JG 301 where he was assigned to the Geschwaderstab. With this unit he flew the Focke-Wulf Ta 152 for the first time at Soltau-Hannover. He gained several successes over Russian fighters around Berlin flying the Ta-152 in the dying days of the war. On 20 April 1945, Walter Loos was awarded the Ritterkreuz for 36 victories.
Walter Loos flew 66 combat missions to achieve 38 confirmed victories and 8 unconfirmed. 30 enemy aircraft were claimed on the Western Front, 22 of them four-engined bombers. He himself was shot down nine times.