Entering the imperial foreign service as attaché in Paris (1877), Aehrenthal subsequently worked at the Austrian Foreign Office and later was appointed diplomatic counselor at St. Petersburg (1888), minister plenipotentiary to Romania (1895), and ambassador to Russia (1899). In 1906 he replaced Count Agenor Gołuchowski as foreign minister. As aggressive as his predecessor was restrained, he revived the dormant foreign policy of the empire.

Aehrenthal's proclamation of the annexation of Bosnia and Hercegovina (October 1908) raised the threat of war with Russia (whose foreign minister, Count Aleksandr Izvolsky, felt himself deceived in his negotiations with Aehrenthal), inflamed the Austrophobe passions of Serbia, and incurred international censure. He consistently opposed suggestions of preventive war against Italy and Russia, however, and sought to reestablish good relations with Italy, Austria's nominal ally, by supporting Italian imperialist ambitions in Libya (1911). The generally assertive course of his foreign policy also led to a cooling of relations with Germany.