After the outbreak of the Napoleonic Wars and creation of the Duchy of Warsaw in 1808, Joanna Żubr with her husband, Michał, left Austrian-ruled Volhynia.

Soon she enlisted in the 2nd Infantry Regiment (4th company, 2nd battalion) as a private, hiding that she was a woman from both her superiors and fellow soldiers.

In 1809, Joanna took part in the Galician Campaign, distinguishing herself in the Battle of Zamość on 19 May that year.

Her husband was an ensign in the same regiment and Joanna Żubr was promoted to sergeant, as the first woman in the Polish Army.

In the summer of 1813, weeks after Prince Józef Poniatowski's forces had abandoned Kraków, she reached Polish units in Saxony and served with distinction until the signing of the Treaty of Fontainebleau and the end of the war.