He set out on a Grand Tour through France in 1550 and began to build up a personal collection of books, the basis of the later Herzog August Library in Wolfenbüttel.

At the instigation of his father, Julius was appointed Prince-Bishop of Minden, by the cathedral chapter on 23 April 1553, succeeding Franz von Waldeck.

In 1552 Julius' father Duke Henry V had joined a princely alliance against the not less warlike Hohenzollern margrave Albert Alcibiades of Brandenburg-Kulmbach.

During the Second Margrave War, both sides met in the 1553 Battle of Sievershausen, where both Julius' elder brothers were killed in action.

Suddenly, he became heir to the principality – for the distress of his father, who noted his feeble constitution and his sympathies for French culture and the Protestant faith.

He also founded a militia — every head of household was required to own a weapon and participate in military training — and reformed the court system.

[1] Julius also had the Dutch architect Hans Vredeman de Vries lay out a network of grachten in his Wolfenbüttel residence.

Upon the death of his Welf cousin Duke Eric II of Brunswick-Lüneburg in 1584, Julius inherited the Principality of Calenberg.