Günther Josten joined the Luftwaffe in January of 1940, and was transferred to the Jagdgruppe Drontheim on 1 November 1941. While serving there, Josten was tasked with defending German ships anchored in the Norwegian fjords from RAF attacks, while providing escort for bombers attacking Allied convoys in the North Sea.

At the end of August 1942, he was sent to Jesau and posted to 3./JG 51. His new unit, the first Gruppe of JG 51, had arrived to Jesau in the middle of the month. Here they replaced their rather worn-out and by this time obsolete Bf 109 F-1 and F-2 for the Focke Wulf 190 A. No less than 43 Fw 190 A-1, A-2 and A-3 types were taken over by the unit.

During early September, flight training on the new type was continued in Jesau before the order to relocate the Gruppe to Russia was given. On September 10th, the unit was moved to the east, flying over Riga and Rjelbitzi and arriving the same day at Ljuban, which was assigned to I./JG 51 as a new base of operations. A total of 36 machines had arrived by the evening. The first Gruppen was here subordinate to the Stab./JG 54, and tasked to protect the air-space above the Northen sector of the Eastern Front.

Flying their first missions on 21 September over Luga, until eight days later when on the unit flew along the Newa river and conducted fighter sweeps in the Leningrad and Dubovka area. Here I./JG 51 would claim their first victory flying the Fw 190, when their Gruppenkommandeur, Hptm. Heinrich Krafft, shot down a I-16 for his 62nd abschüss. The Gruppe also experience their first loss this day when Uffz. Erwin Pflaum was shot down and wounded in his Fw 190 A-3.

The next day the Gruppe would claim another four-enemy aircraft, this time without any losses. Uffz. Günther Josten however, would not claim his first victory until 23 February 1943. It was not until the summer months of 1943, that he would start to score numerous victories, 19 of them during July and another 30 in August. In September 1943, Josten would repeat his successes by downing another 26 VVS aircraft. Among these, he was able to score several multiple victories, seven kills on the 7th and another eight on the 15th, his 71st to 78th Abschüsse.

After reaching 82 victories, Josten was posted to Fursten-feldbrück, where he served as an instructor until he returned to the Eastern Front on 5 February 1944.