[3] In around 1516, a community of Protestants known as the Unity of the Brethren (Unitas fratrum) were expelled from the Bohemian lands by King Vladislaus II and settled in Leszno.

Leszno was a private town, administratively located in the Wschowa County in the Poznań Voivodeship in the Greater Poland Province.

Their numbers grew with the inflow of refugees from Silesia, Bohemia, and Moravia during the Thirty Years War.

In 1631, Leszno was vested with further privileges by King Sigismund III Vasa, who made it equal with the most important cities of Poland such as Kraków, Gdańsk and Warsaw.

By the 17th century, the town had a renowned Gymnasium (school), which was headed by Jan Amos Komenský (known in English as Comenius), an educator and the last bishop of the Unity of the Brethren.

[5] The era of Leszno's prosperity and cultural prominence ended during the Second Northern War, when the town was burnt down on 28 April 1656 by Polish forces faithful to King John Cassimir Vasa in retribution for letting in Swedish forces by protestant majority of burghers.

[6] Quickly rebuilt afterwards, it was set on fire again during the Great Northern War by Russian forces in 1707 and was ravaged by plague in 1709.

The Leszczyński family owned the city until 1738, when King Stanislaus I Leszczynski sold it to Aleksander Józef Sułkowski following his abdication.

In 1807 it was taken by Napoleon's Grand Armee and included within the newly established but short-lived Polish Duchy of Warsaw.

[8] Afterward the city became part of the newly established Second Polish Republic under the Treaty of Versailles, with effect from 17 January 1920.

[8] During the joint German-Soviet invasion of Poland, which started World War II in September 1939, the town was annexed by Nazi Germany and incorporated into Reichsgau Wartheland.

[12] Poles who were initially imprisoned in Leszno were also murdered in nearby towns and villages of Poniec, Osieczna, Włoszakowice and Rydzyna.

[15] Poles were held there several days, their money, valuables and food were confiscated, and then they were either deported to Tomaszów Mazowiecki or Łódź in German-occupied central Poland or sent to local German colonists or to Germany as slave labour.

Leszno has an oceanic climate (Köppen: Cfb) although notably with warm summer continental characteristics (Dfb), typical of inland west and south polish.