He was first mentioned in 1355. When his father died in 1379, he ruled
Mecklenburg together with his brothers Heinrich and Albrecht. However,
Heinrich died in 1383 and Magnus I on 1 September 1384 after his return
from Aachen where he had been for health reasons. He had been engaged
first to a daughter of the Prince of Werle-Goldberg, then to Catharina,
daughter of the Prince of Werle-Gustrow, then to Jutta of Saxe-Lauenberg,
then finally to Elisabeth of Pommern whom he married in 1362. They became
the parents of Johann IV.
Best wishes
Leo van de Pas