He was the eldest son of Henry Borwin II, Lord of Mecklenburg.

However, Saxe-Lauenburg and Holstein then began claiming suzerainty over Mecklenburg and he had to wage war against them to retain his sovereignty.

In 1255, his brother Pribislaw I lost his Lordship of Parchim-Richenberg after a conflict with Bishop Rudolph I of Schwerin.

Pribislaw was taken prisoner by the bishop and his brothers divided his territory between themselves, with John I receiving Sternberg.

John I supported the Church and the settlement of ethnic Germans in Mecklenburg, which was still predominantly Slavic during his reign.