How many years did Genghis Khan rule as Great Khan?

Genghis Khan ruled as Great Khan for about 21 years, from 1206 until his death in 1227.

Before 1206, he was known as Temüjin and spent decades building power through alliances, warfare, and the incorporation of rival Mongol tribes. At a great assembly, or kurultai, in 1206, the Mongols recognized him as their supreme ruler and bestowed the title Genghis Khan.

During his 21-year reign, he unified the Mongol steppe and launched campaigns against the Western Xia, the Jin dynasty in northern China, and the Khwarezmian Empire in Central Asia and Iran. His empire expanded enormously, but much of the conquest continued under his sons and successors after his death.

The count is conventionally given as 21 years because it uses the standard reign dates of 1206–1227. Exact day-and-month calculations are less useful because the accession date is not securely fixed and Genghis Khan’s death date is variously reported as 18 or 25 August 1227. The 21-year answer is therefore the fair canonical choice.