John was the elder son of Duke Albert I of Saxony and his third wife Helen, a daughter of Otto the Child.
In the imperial election in 1273, Albert II represented the jointly ruling brothers.
In 1270, John married Ingeborg (c. 1253–30 June 1302), possibly a daughter of Birger Jarl or King Erik Eriksson of Sweden.
Later John I entered the Franciscan monastery in Wittenberg, becoming its warden (guardian), which position he held until he died.
John I's sons and their uncle Albert II continued the joint rule in Saxony.