He had to serve with Kampfstaffel 20 of Kampfgeschwader IV as an observer before he could earn his pilot's badge. Transferring to scout aircraft, he was posted to Oswald Boelcke's Jasta (Jagdstaffel) 2, where he flew as Manfred von Richthofen's wingman. At the age of only 18, he scored his first victory on November 27 1916. Flying an Albatros D.III scout aircraft decorated with an Iron cross and heart motifs (for good luck), he achieved 38 credited victories.
He was subsequently promoted to temporary commands at Jastas 5, 29, and 14, before moving to a permanent command at Jasta 10 as part of Richthofen's Jagdgeschwader I (JG I) (or "Flying Circus" as it later became known to the Allies). Having tested one of the F.1 prototypes (103/17, Wk. Nr.1730) of the Fokker Dr.I triplane scout for Anthony Fokker, Voss evidently adapted his flying style to the rotary engined triplane, being credited with a further 10 victories with this new aircraft. He adorned the cowling of his new aircraft by painting two eyes, eyebrows, and a moustache (a face motif thought by some to derive from Japanese kites). Voss was known for being a loner and an inspirational, rather than effective, leader (modern writers often describe him as 'mercurial').
He was finally shot down after single-handedly engaging up to eight Royal Aircraft Factory SE5s of 60 and 56 Squadrons of the Royal Flying Corps on September 23 1917 over Poelcappelle. Although the SE5s were flown by some of the RFC's best aces (James McCudden, Richard Maybery, Keith Muspratt, Reginald Hoidge, Arthur Rhys Davids and Hammersley), by exploiting the triplane's superior rate of climb and its ability to slip turn (using the rudder to turn quickly, which the triplane was very good at), Voss continually outflew his opponents. He was able to swing around at high speeds and attack those behind him, practically flying backwards. However, after flying past McCudden in a head-on confrontation, the rear of Voss's Fokker was exposed to Rhys Davids of 56 Squadron and was struck by multiple rounds. Voss's previously-masterful flying gave way to a shaky, limping retreat. McCudden watched from high altitude as Voss went into a steep dive from which he never recovered. The uncharacteristically passive final moments of Voss's flight and his seemingly-suicidal dive suggest that he had been injured by the final volley from Rhys David and was struggling to maintain consciousness. His aircraft crashed near Plum Farm north of Frezenberg in Belgium. Only the rudder, cowling, and parts of the undercarriage were salvaged and the aircraft was the subject of a report by 2nd Lieutenant G. Barfoot-Saunt.
One of the British pilots he fought that day, Major James McCudden, a recipient of the Victoria Cross who would become a leading English ace of the war, expressed sincere regret at his death: "His flying was wonderful, his courage magnificent and in my opinion he was the bravest German airman whom it has been my privilege to see fight."
Voss' decorations and awards include: the Iron Cross 1st and 2nd Class, the Knights Cross with Swords of the Hohenzollern House Order, and the Orden Pour le Mérite (the "Blue Max"). His final tally was 48 confirmed victories.