Mierosławski took part in the November Uprising of the 1830s, and after its failure he emigrated to France, where he taught Slavic history and military theory.

Mierosławski attended schools in Łomża and Kalisz, and at the age of 15 he became a podchorąży (officer cadet) in the 5th Infantry Pułk.

During the November Uprising, when Poles rose against the Russian forces in Congress Poland, he was a lieutenant serving under General Samuel Różycki.

In 1839–1840 he gave lectures on the history of Slavic people in French Historical Institute in Paris; he was also considered by many among the Polish emigrants as a knowledgeable tactician and military strategist after the publication of a history of the November Uprising in Poland, Histoire de la revolution de Pologne (Paris, 1836–38).

In the next few years, he would act as the commander of the Greater Poland Uprising of 1848, chief of staff of the revolutionary Italian army in Palermo (Sicily) fighting against Bourbons (December 1848 – April 1849) and then commander of German insurgent units in Baden and in the Electorate of the Palatinate during the revolutions of 1848 in the German states.