Słowikowski joined the Polish Army in 1918 and fought in the Polish-Soviet War of 1919-1921. After completing advanced military studies in 1925, he worked at the Polish Ministry of Defense. For four years he served on Marshal Józef Piłsudski's staff. In 1937 he was transferred to the Ministry of Foreign Affairs and appointed secretary at the Polish Consulate in Kiev in Soviet Ukraine. His actual function, however, was collecting intelligence in southern Russia. British historian M. R. D. Foot, in his foreword to Słowikowski's book In the Secret Service, writes: