In interwar Poland he was co-editor of Ilustrowany Kurier Codzienny (Illustrated Daily Courier), where his beat was German politics and policy.
During World War II he escaped from Nazi occupied Poland and worked for publications of the Polish government in exile, "Ku Wolnej Polsce" (For a Free Poland), "Orzeł Biały" (The White Eagle), "Parada" (Parade).
[1] While at the time Poland was an authoritarian communist state, controlled by the Soviet Union, Mieroszewski and Giedroyc, in the pages of the journal, sought to articulate a political programme for what they envisioned as a future independent Polish state and its relations with its former Kresy territories, annexed by Soviet Russia, known as the Gedroyc-Mieroszewski doctrine.
Mieroszewski was not only a dedicated socialist, but was strongly opposed to communism and the Soviet domination of Eastern Europe.
A crucial, and at the time unique, consideration of the Kultura programme was the Polish relationship with the national aspirations of the country's former minorities, the Belarusians, Lithuanians and Ukrainians.