In July 1917, during the Legions' "Oath Crisis" (precipitated by a demand from Germany and Austro-Hungary that the Polish Legionnaires swear loyalty to them), Jędrzejewicz was imprisoned by the Germans.

On 24 April 1920, Jędrzejewicz, now a captain, signed a military convention with Ukraine's Ataman Semen Petlura which paved the way for the Polish Army's 1920 Kiev Expedition.

On 22 January 1934, he was appointed Minister of Religious Denominations and Public Education in the government of his brother, Premier Janusz Jędrzejewicz (1885-1951), serving on as well in the premierships of Leon Kozłowski and Walery Sławek.

When World War II broke out in September 1939, Jędrzejewicz helped evacuate the treasury of the Fund for National Defense, which in February 1940 was delivered to General Władysław Sikorski's Polish government-in-exile in Paris.

Due to the anti-Piłsudskiite policies of General Sikorski (whose prewar career had been derailed by differences with Piłsudski), Jędrzejewicz was prevented from serving now with the Polish Armed Forces in exile.

Jędrzejewicz published some 300 scholarly papers in history and several major books, including Poland in the British Parliament, 1939-45 and a two-volume Kronika życia Józefa Piłsudskiego (Chronicle of the Life of Józef Piłsudski).