[2] Duke Magnus was born at the Copenhagen Castle in 1540 as the second son of King Christian III of Denmark and Norway and Dorothea of Saxe-Lauenburg.

The same year, the prince-bishop of Ösel-Wiek and Courland Johannes V von Münchhausen in Old Livonia sold his lands to King Frederick II for 30,000 thalers.

To avoid hereditary partition of his lands, King Frederick II gave that territory to his younger brother Magnus on condition that he renounced his rights to succession in the duchies of Schleswig and Holstein.

[3] During the Livonian War, on 10 June 1570, Duke Magnus arrived in Moscow with the approval of his older brother, where he was crowned King of Livonia by Ivan IV of Russia.

[1][4] In 1577, having lost Ivan's favor and receiving no support from his brother, Magnus called on the Livonian nobility to rally to him in a struggle against foreign occupation.