Próchnik was born in Lwów, Austrian partition (now, Lviv, Ukraine) on 21 August 1892 to a middle class Jewish family.

[1] Before World War I he joined the pro-independence paramilitary Polish organization, the Union of Armed Struggle.

[1] In interwar Poland, Próchnik became an activist of the Polish Socialist Party and supported initiatives designed to improve the situation of the country's working class.

[1] He steadily drifted to more extreme left position, supporting cooperation with the communists.

His attempt to enter the academic life ended when his application for a position in the University of Warsaw was rejected, due to his left wing views being unpopular among the right wing faculty there (despite a support from his candidature from professor Wacław Tokarz.

[1] During World War II, in occupied Poland, Próchnik continued his political activities, supporting a PPS-affiliated underground printing press (Barykada Wolności).

In his historical works, Próchnik was a strong adherent to Marxist views, and supported the dialectic materialism perspective.