While Erik has been regarded as intelligent and artistically skilled, as well as politically ambitious, early in his reign he showed signs of mental instability, a condition that eventually led to insanity.

Erik's first teacher was the learned German Georg Norman, whose services were shortly thereafter needed elsewhere within the Swedish state.

Against his father's wishes, Erik entered into marriage negotiations with the future Elizabeth I of England and pursued her for several years.

Erik also made unsuccessful marriage proposals to, among others, Mary, Queen of Scots (1542–1587), Renata of Lorraine (1544–1602), Anna of Saxony (1544–1577) and Christine of Hesse (1543–1604).

Erik learned of his father's death as he was on the point of embarking for England to press his suit for the hand of Queen Elizabeth I.

He and his brother Charles adopted regnal numbers according to Johannes Magnus's partly fictitious history of Sweden.

John was the Duke of Finland and was married to a Polish princess, against Erik's wishes, which made him friendly with Poland.

John pursued an expansionist policy in Livonia (now Estonia and Latvia), in breach of the Arboga articles, which led to contention between the brothers.

Unlike his father, who had been satisfied with ruling an independent state, Erik tried to expand his influence in the Baltic region and in Estonia, beginning the process that resulted in Sweden becoming a great power in the 17th century.

This marriage was solemnized in July, Karin was crowned queen, and their infant son Gustav pronounced prince-royal.

A document signed by his brother, John III, and a nobleman, Bengt Bengtsson Gylta (1514–74), gave Erik's guards in his last prison authorization to poison him if anyone tried to release him.