In 1919, as a 17-year-old volunteer he participated in the Polish–Soviet War, first, as an uhlan of the 10 Polish Cavalry Regiment, and then of the 13th Regiment of Wilno Uhlans.
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In 1942, he witnessed the Ponary massacre of some 100,000 mostly Polish Jews by German SD, SS and the Lithuanian Nazi collaborators Ypatingasis būrys, which he described in his 1969 book Nie trzeba głośno mówić (One Is Not Supposed to Speak Aloud).
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Similar to other young veterans of the war who entered university without their Matura graduation, also Mackiewicz started his favourite subject of biological sciences at the University of Warsaw and then upon moving to Vilnius continued studies at the Vilnius University, but he never graduated with a degree.
Józef Piłsudski | Józef Ignacy Kraszewski | Józef Simmler | Józef Poniatowski | Jozef Gabčík | Józef Michalik | Józef Andrzej Załuski | Józef Dominik | University of Pavol Jozef Šafárik | Józef Wybicki | Jozef Štafura | Józef Razowski | Józef Maria Hoene-Wroński | Józef Mackiewicz | Józef Kustroń | Jozef Israëls | Józef Dwernicki | Jozef Cleber | Józef Brudziński | Jozef Brudzinski | Józef Bem | Józef Zajączek | Józef Wojciechowski | Jozef Tomko | Józef Sowiński | Józef Potocki | Józef Pietrończyk | Józef Peszka | Józef Olszyna-Wilczyński | Jozef Murgaš |
Information about the massacre began to spread as early as 1943, due to the activities and works of Helena Pasierbska, Józef Mackiewicz, Kazimierz Sakowicz and others.