Mertens was born on 11 November 1917 in Essen. He joined the label=none in 1937. By the outbreak of war on 1 September 1939, Mertens had been commissioned as leutnant and was a fighter pilot flying the Messerschmitt Bf 109 with No. I Gruppe of Lehrgeschwader 2LG 2. With this unit he participated in Operation Case Yellow during the Battle of France and on 19 May 1940, shot down a Royal Air ForceHurricane fighter his first combat victory. On 3 June 1940, he shot down a French Curtiss fighter his second victory. Mertens transferred to Jagdgeschwader 3 flying with their 9 Staffel in the Battle of Britain. His unit provided escort cover for German bombers attacking England and on 26 October 1940, he shot down another Hurricane fighter.
Eastern Front
During Operation Barbarossa, Mertens shot down DB-3 bomber on 26 June 1941, and then during July 1941 he shot down eleven more Soviet aircraft including three in a day on 16 July 1941. He was awarded the Iron Cross 1st Class. On 23 July 1941, he was appointed squadron commander of his squadron, 9 Staffel. Mertens shot down seven more aircraft in August 1941, one more in September, and another in October 1941 In February 1942, Mertens transferred to the Staff of Jagdgeschwader 3 and was promoted oberleutnant before joining the staff of III Gruppe, Jagdgeschwader 3. He was awarded the German Cross in gold on 27 May 1942, or 8 June 1942. Between 6 April 1942, and 26 July 1942, he shot down fifteen more Russian aircraft, primarily Lavochkin-Gorbunov-Gudkov LaGG-3 and Polikarpov I-153 fighters and Ilyushin DB-3 bombers, bringing his tally to forty confirmed victories. In late July 1942, Mertens was appointed to command the 1 Staffel of Jagdgeschwader 3 and promptly shot down seven more Russian fighters and bombers during the next four weeks. A photograph in Luftwaffe Rudder Markings 1936-45 taken on 22 July 1942, in the Stalingrad sector shows him posing with the tail of his Bf 109 which is marked with forty-seven victory tabs. He was awarded the Knight’s Cross of the Iron Cross on 4 September 1942, the recommendation at the time stated that he had achieved fifty victories in air combat. His award is well documented. Following this he was temporarily appointed Gruppenkommandeur of I Gruppe of Jagdgeschwader 3 until November 1942. Documentation concerning Mertens service in the next eight or nine months from the time of his award until July 1943 is scarce and no details of his actions as a fighter pilot are known.
By July 1943 he was back in action flying with 1 Staffel Jagdgeschwader 3 on the Western Front flying in the Defence of the Reich against the daylight attacks on German home targets primarily by 8th USAAF formations. On 30 July 1943, he shot down a B-17Flying Fortress and on 17 August 1943, another B-17 Flying Fortress. On 23 September 1943, Mertens was appointed commander of 4 Staffel of label=none to complete the operational training phase for young pilots arriving from fighter pilot school Jagdfliegerschule. He was promoted Hauptmann on 1 October 1943, and on 8 January 1944, returned to fully operational duties when he was appointed to command the 7 Staffel Jagdgeschwader 3. On 20 Feb 1944, Mertens shot down a B-17Flying Fortress and then made a ramming attack on a B-24 Liberator. In this attack his Bf 109 was wrecked, he was wounded, and bailed out. This was to be his last operational mission. While recovering, Mertens was briefly appointed to command 1 Staffel of label=none in April 1944 until he assumed temporary command of I Gruppe Jagdgeschwader 3 and then joined the staff of Adolf Galland General of Fighter pilots. From November 1944 Mertens held posts within label=none and finally the operational training establishment of Jagdgeschwader 1. At the end of the war he was credited with about 750 combat missions and fifty or fifty-four confirmed air victories and the sinking of a Russian gun boat. Obermaier records sixty victories in the East and between sixteen and twenty four-engined bombers shot down in the West.
