He was born to a Polish family in
Bromberg (Bydgoszcz). Some sources state his forename as Sylvester, but Mieczysław was his first name and was the one used by him. He intended to study medicine, but in November 1915 he was mobilized into the
German army. Initially he served in the
infantry on the Western Front with Grenadier Regiment 110 and Infantry Regiment 87. After completing officer training in April 1917 he became a Leutnant. He was slightly injured commanding a company in the 1st battalion of the 1st Nassau Infantry Regiment Nr. 87, and was awarded the German
Iron Cross 2nd class. Then he asked to be transferred to the air service, and in November 1917 he was sent to flying school. After completing his flying training in
Posen (Poznań), and then a fighter pilot course in Nivelles, he was assigned to
Jagdstaffel 31 (
Jasta 31) in St. Loup in
Champagne on 7 June 1918. Initially he flew the
Albatros D.V and later the
Fokker D.VII fighter. His combat career was short but successful. He shot down his first claim, a SPAD fighter, on 16 July 1918 and was given a fighter pilot badge. On 18 September he shot down an
S.E.5a and a
DH.9 bomber, then another DH.9 on 25 September. The following day he shot down a
Sopwith Camel of No. 208 Squadron, and on 30 September he claimed his sixth (and last) kill, an S.E.5a. from No. 92 Squadron. On 2 October he was wounded during combat with Camels of
No. 46 Squadron RAF, but managed to land and was hospitalised. His probable victors were Captain
Donald MacLaren, James Leith and Cyril H. Sawyer. Garsztka was the only pilot of Jasta 31 to become a
fighter ace. On 25 September 1918 he was awarded the German
Iron Cross 1st Class. ==Polish service==