 Frederick was the eldest son of Duke Magnus II of Brunswick-Lüneburg, Prince of Wolfenbüttel, and his consort Catherine of Anhalt-Bernburg.A minor upon his father's death in 1373, he was not declared to have reached the age of majority until 1381, while Duke Otto of Brunswick-Göttingen acted as his guardian.Like his father, he was thrust into the Lüneburg Succession War, which he and his brothers attempted to end in 1373 by a treaty with the Ascanian dukes of Saxe-Wittenberg.