1745) in the Vistula delta Mennonite settlement of Petershagen, Prussia, located about 35 km east of the city of Danzig (Gdańsk).
In 1792, he moved to Neuenhuben, a village just east of Danzig, where he joined a newly established Werder Gemeinde, a Mennonite splinter church.
About three months after his first wife died, Reimer remarried to Helena von Riesen (1787–1846), together they had ten children, of which three sons and two daughters survived to adulthood.
In 1804, he led a group of about 30 adults in an immigration to south Russia, settling in the newly opened Mennonite colony, Molotschna, in the spring of 1805.
These later united to form the Kleine Gemeinde, which was recognized by the Russian government and granted the same rights and privileges of the main Mennonite group.
A faction within the church apparently dwelt on guilt and fear and attempted a daily routine of extreme asceticism and self-inflicted punishment.
Direct descendants of Klaas Reimer include authors Miriam Toews, David Bergen, Casey Plett and Andrew Unger.