He was born on 20 November 1884 in Gniewczyna Łańcucka. He attended junior high school in Sanok. He received a degree in philosophy from the Jagiellonian University. In 1911, under the pseudonym Jan Brzoza, he published Zagadnienie polityki niepodległości. He was one of the leading economists of the so-called Second Polish Republic: among others, he cooperated with Władysław Grabski on the 1924 currency reform, he co-founded Bank Polski SA, acting as its deputy president until 1930. He also engaged in political activities, also within the Morges Front. During the interwar period, he published two important books: Człowiek w dziejach. Jednostka-państwo-naród (1936) and Totalizm czy demokracja w Polsce (1938), where he presented his vision of the national policy, the so-called natiocratism, embedded in the realities of the growing popularity of dictatorial systems: fascism, Nazism and communism. He was an opponent of statism, and saw its manifestations also in the economies that were considered capitalist. During World War II, he headed Bank Emisyjny w Polsce (Issuing Bank in Poland), established upon Hans Frank’s decision, which issued money for the Generalgouvernement (colloquially called młynarki). The occupation authorities agreed to the conditions put forward by Młynarski i.e. that the inscriptions on the banknotes should be in Polish and the word Polska (Poland) should appear in the name of the bank. The acceptance of the German proposal was preceded by his consultation with the underground forces and  General Władysław Sikorski’s government, where he obtained endorsement. Nonetheless, after the war, because of being the president of Bank Emisyjny w Polsce, he was accused of collaborating with the Nazis and although the investigation against him was discontinued in 1946, this hindered Młynarski’s academic career in communist Poland. He could not get a professorship but, instead, he became the director of the library of the Cracow School of Economics (Wyższa Szkoła Ekonomiczna). He died on 13 April 1972 in Kraków.