Géza II was the oldest son of Béla the Blind and his wife, Helena of Serbia. When his father died, Géza was still a child and he started ruling under the guardianship of his mother and her brother, Beloš. A pretender to the throne, Boris Kalamanos, who had already claimed Hungary during Béla the Blind's reign, temporarily captured Pressburg (now Bratislava in Slovakia) with the assistance of German mercenaries in early 1146. In retaliation, Géza, who came of age in the same year, invaded Austria and routed Henry Jasomirgott, Margrave of Austria, in the Battle of the Fischa.

Although the German–Hungarian relations remained tense, no major confrontations occurred when the German crusaders marched through Hungary in June 1147. Two months later, Louis VII of France and his crusaders arrived, along with Boris Kalamanos who attempted to take advantage of the crusade to return to Hungary. Géza joined the coalition that Louis VII and Roger II of Sicily formed against Conrad III of Germany and the Byzantine Emperor Manuel I Komnenos. The ancestors of the Transylvanian Saxons came to Hungary during Géza's reign. Western European knights and Muslim warriors from the Pontic steppes also settled in Hungary in this period. Géza even allowed his Muslim soldiers to take concubines.

Géza intervened at least six times in the fights for Kiev on behalf of Iziaslav II of Kiev either by sending reinforcements or by personally leading his troops to the Kievan Rus' between 1148 and 1155. He also waged wars against the Byzantine Empire on behalf of his allies, including his cousins, rulers of the Grand Principality of Serbia, but could not prevent the Byzantines from restoring their suzerainty over them. Conflicts emerged between Géza and his brothers, Stephen and Ladislaus, who fled from Hungary and settled in Emperor Manuel's court in Constantinople. Géza supported Frederick I, Holy Roman Emperor, against the Lombard League with auxiliary troops between 1158 and 1160.