Eric was born c. 1290 as the only legitimate son of Duke Valdemar IV of Schleswig in his first marriage with Elisabeth of Saxe-Lauenburg, a daughter of John I, Duke of Saxony. Already during the lifetime of Valdemar IV, he acted as his father's coruler and used the title of duke.
Duke of Schleswig
At the death of Valdemar IV in the spring of 1312, he immediately succeeded his father as Duke of Schleswig. Shortly afterwards, he participated in the expedition of his feudal overlord, King Eric VI of Denmark, to Rostock. During this expedition, on 30 June in the camp outside Warnemünde, he was formally installed as Duke.
When King Eric VI of Denmark died in 1319, Duke Eric was briefly candidate to the vacant throne of Denmark. Ultimately, Eric VI's brother, Christopher, was elected King. However, part of the price Christopher had to pay to accede to the throne, was to accept the right of Duke Eric to the fief of Langeland.