He was born in Hitzacker, then the residence of his father Duke Augustus the Younger of Brunswick-Lüneburg (1579–1666) and his second wife Princess Dorothea of Anhalt-Zerbst (1607–1634).The next year his father, at the age of 55, assumed the rule in the Principality of Brunswick-Wolfenbüttel after his Welf cousin Duke Frederick Ulrich had died childless.Anthony Ulrich was the second surviving son of the ducal couple; he and his siblings received a comprehensive education at the Wolfenbüttel court by scholars like Justus Georg Schottel and Sigmund von Birken, as well as by his art-minded stepmother Elisabeth Sophie of Mecklenburg (1613–1676).Rudolph Augustus had more interest in hunting and his library than in government affairs and left most decisions to his brother; in 1685, he officially made Anthony Ulrich a coregent with equal rights.The young prince united the forces of the Welf principalities to combat the rebellious City of Brunswick, whose citizens finally had to accept the ducal overlordship in 1671.He continued to settle various disputes with his Hanover cousin George Louis, who in 1705 also inherited Lüneburg, until a final agreement between the two sister principalities was reached in 1706.