In 1164, the Byzantine Emperor Manuel I Comnenus concluded a treaty with Béla's brother, Stephen III, by which Béla was given the Croatian and Dalmatian territories and sent to Constantinople to be educated in the court of the emperor. Manuel, who had no sons of his own, intended the youth to succeed him and thus betrothed him to his daughter, Maria Comnena. Béla received a Greek name, Alexius, and the title of despot. When a son was born to the emperor this engagement was broken, but another marriage was arranged between Béla and Agnes de Chatillon, a princess of Antioch. She was also the half-sister of Empress Maria.
Béla succeeded his brother King Stephen III and was crowned under the influence of Emperor Manuel. As the new king, Béla adopted Catholicism and selected his son Emeric as his successor. He was a powerful ruler, and his court was counted among the most brilliant in Europe.
Béla was a warrior by nature and training, and the death of Emperor Manuel in 1180 left him free to expand Hungarian power in the Balkans. His attempt to recover Dalmatia led Hungary into two wars against Venice which ultimately faltered. He also aided the Serbs against the Byzantine Empire. At the time of his death Béla was assisting Emperor Isaac II Angelus in a war against Bulgaria. He was succeeded by both of his sons in turn, Emeric and Andrew.
Béla is interred at the Mathias Church Cemetery in Budapest, with his second wife Agnes. Through his son Andrew II, he is the ancestor of Edward III of England through Isabella of France. He is therefore an ancestor to the present-day British royal family including Queen Elizabeth II.