Constantine Diogenes is the first notable member of the noble Diogenes family with origins from Cappadocia, which played an important role in 11th-century Byzantium. Diogenes began his career as a commander of one of the western "tagmata" during the reign of Basil II (r. 976–1025), in the latter's campaigns against Bulgaria. In 1014, he participated in the decisive Byzantine victory at the Battle of Kleidion (July 29), and he subsequently succeeded Theophylact Botaneiates as commander ("doux") of Thessalonica with the rank of "patrikios", making him the second-most senior general of the Empire in the Balkans after David Arianites. Following the death of Tsar Samuel of Bulgaria in October, Diogenes and Nikephoros Xiphias were dispatched to the region of Moglena as the vanguard of the emperor and the main army. During this campaign, Diogenes constructed the fortress of Mylobos, as attested in a founder's inscription.