A medieval town, together with a nearby settlement, had already existed in this area in the 12th century, and it was granted municipal rights before 1318. King Casimir III of Poland presented Maćko Borkowic with the town, so he could build his castle there. Later on, the town changed its owners numerous times. In the 16th century, Koźmin belonged to the property of the Górek family, and after to the Sapieha family. In the period of the First Rzeczpospolita, Koźmin was considered one of the most powerful towns in the region of Wielkopolska. In the 16th century, a prospering drapery center was functioning in the town. In the 17th century, Koźmin became a center of the Reformation (the Unity of Brethren opened a printing house in Koźmin in 1561).

The Labor-Soldier Council was established there in November 1918, and a soldier company – after the breakout of the Wielkopolskie Uprising. In 1875, a railway was built, an event which contributed to rapid town growth. In the period of 1887-1932 Koźmin was the county town (the county of Koźmin).

During the Second World War, from the 6th of September 1939 until the 23rd of January 1945 Koźmin remained under the German occupation. In the period of 1975-1998, the town belonged administratively to Kaliskie Province.Before 1875 and after 1999 – to the county of Krotoszyn.

On the 1st of January 1997 the adjective “Wielkopolski” was added to the town name. Since that moment, the official name of the town is Koźmin Wielkopolski.