Temüjin built the Mongol Empire through a combination of personal alliances, military victory, and the reorganization of steppe society. His early struggle for followers involved cooperation and conflict with figures such as Toghrul of the Kereit and his former companion Jamukha. Success against rival groups, including the Tatars, Kereit, and Naiman, gradually gave him supremacy. In 1206 an assembly recognized him as Genghis Khan. Accounts of these events, particularly the Secret History of the Mongols, preserve valuable traditions but were shaped by the dynasty's later perspective.

Genghis Khan organized followers into military units that weakened older tribal loyalties and linked commanders directly to his authority. Discipline, intelligence gathering, coordinated cavalry operations, and the incorporation of defeated peoples strengthened his forces. Uighur writing and personnel from neighboring societies supported administration. His commands and customary rules are often described as a unified law code called the Yasa, but the existence and form of such a comprehensive written code remain disputed.

Expansion beyond the steppe began with pressure on Western Xia and war against the Jin dynasty in northern China from 1211. Mongol forces captured Zhongdu, the Jin capital, in 1215, but conquest of the dynasty was completed only under his successors. Engineers, infantry, and administrators recruited from conquered societies helped extend warfare beyond the possibilities of steppe cavalry alone. In 1219 Genghis Khan invaded the Khwarazmian Empire after the killing of a trading delegation and the breakdown of diplomatic relations. His armies took major cities in Central Asia and advanced into Afghanistan and Iran.

The western campaigns involved massacres, enslavement, deportation, and widespread destruction. Contemporary numerical claims are often exaggerated, but the scale of violence and disruption is not in doubt. Submission sometimes spared communities the worst treatment, while resistance could provoke devastating reprisals. Genghis Khan returned east and died in 1227 during the final campaign against Western Xia. His burial place is unknown, and explanations of his death vary. He designated Ögödei to succeed him, while territories and followers were distributed among family members. His descendants greatly enlarged the empire, so later conquests should not all be credited to his personal command.