The Mongols were nomadic herders and hunters who spent their lives in the saddles of their steppe ponies. They learned to ride and use weapons, especially the composite bow, at an early age. For hunting and war, every able-bodied male under the age of 60 years was expected to take part. The armies of the united Mongol tribes consisted of the entire adult male population.

They fought under a strict code of discipline. Booty was held collectively. The penalty was death for abandoning a comrade in battle. This discipline, together with leadership, intelligence-gathering, and organization, raised the Mongol force from a cavalry swarm into a true army.

The Mongol army was organized according to a decimal system, with units of 10, 100, 1000, and 10,000 men. These numbers for units were probably rarely approached due to casualties and attrition. The 10,000-man unit was the major fighting unit, like a modern division, capable of sustained fighting on its own. Individual soldiers identified most with the 1000-man unit of which they were a part, the equivalent of a modern regiment. Original Mongol tribes fielded their own 1000-man units. Conquered peoples, such as the Tatars and Merkits, were broken up and distributed among other units so that they could pose no organized threat to the ruling family.

Genghis Khan created a personal guard unit of 10,000 men. This unit was recruited across tribal boundaries and selection was a high honor. In its early stages it served as a form of honorable hostage-holding. It grew into the family household and the source of the growing empire's ruling class.

Mongol soldiers at first received no pay other than booty. Advancement was based on merit. Once the rapid conquests slowed, a new system of pay was put in place. Officers were later able to pass on their posts to heirs.

Each soldier went on campaign with approximately five horses, allowing quick changes and rapid movements. No comparable armies moved as rapidly as the Mongols until the mechanized armies of the twentieth century.

The Mongols fought mainly as light cavalry archers (unarmored), using the compound bow. This was a compact weapon of impressive range and penetration power. They employed Chinese and Middle Easterners as siege engineers. Infantry, garrison troops, and heavy cavalry (wearing armor) that used lances came from the armies of subjected peoples.