Tomaszewska was born in 1814 in Raseiniai, Samogitia, then part of the Russian Empire and previously part of the Polish–Lithuanian Commonwealth. She was born into the Polish-Lithuanian nobility, and was educated at a Benedictine monastery in Kražiai. While she was attending the monastery, a large-scale revolt broke out in Warsaw against the Russian garrison of the city. This revolt grew into the November Uprising of 1830–1831, during which the pro-independence Polish National Government organized local resistance groups to fight against the Imperial Russian Army. Tomaszewska left her monastery and joined one of these groups, equipping herself with a horse, lance, and sabre. She proved a skillful enough combatant to join an organized Polish unit of lancers.