Background
He was the eldest son of Henry Borwin II, Lord of Mecklenburg.
He was the eldest son of Henry Borwin II, Lord of Mecklenburg.
As the eldest son, he received the ancestral homeland. In 1227, he defeated the Danish in the Battle of Bornhöved, thereby relieving Mecklenburg from Danish suzerainty. However, Saxe-Lauenburg and Holstein then began claiming suzerainty over Mecklenburg and he had to wage war against them to retain his sovereignty.
John I supported the Church and the settlement of ethnic Germans in Mecklenburg, which was still predominantly Slavic during his reign.
In 1262, he concluded an alliance with the Guelphs against Denmark. He died in 1264 was buried in the Doberan Minster.