The Mongols adopted a whole range of military technologies and ideas. They began with the usual steppe military system: cavalry armies, with the bow (used while mounted) as the standard weapon, supplemented by swords, maces, lances, etc. for hand-to-hand fighting. Organisation was essentially by clan and tribe; to field a large army required a lot of political activity, to convince various clan/tribe leaders to bring their forces along. Tactics included harassment by mounted archery followed by close fighting if the enemy broke or wavered, feigned retreats, avoiding battle against strong opponents (which could include luring less mobile armies deep into your own territory).

From this base, the Mongols ended up with a system that was in many ways a synthesis of this steppe system with major elements of the military systems of the Chinese. The Mongols were not the first to do this - they more or less adopted the Khitan (Liao) military system, itself a synthesis of the steppe and Chinese systems. What did this entail? From the steppe side came a strong dependence on cavalry and mounted archery, while from the Chinese side came the use of specialised armoured cavalry, and more systematic organisation of military units.

The Jurchen (Jin) had also adopted elements of this combined system, and the Mongols under Genghis (then still Temüjin) fought against the Tatars alongside the Jin, as a junior ally, in 1196, early in Genghis's rise to control of Mongolia. Thus, they had early exposure to this system. The cavalry consisted of 40% heavily-armoured lancers and 60% lighter-armoured mounted archer, often deployed 5 deep in battle. These were supplemented by militia infantry when required. The Jin army, following Khitan practice, was organised decimally, at least nominally, with major units of 1,000 (mengan) composed of 10 units of 100 men (mouko). In practice the units of 100 tended to have about 25 men, and the units of nominally 1,000 had much lower strengths than 1,000. The Jin also had an Imperial Guard unit, originally 3,000 strong when the Mongols began their long war with the Jin, later rising to 5,000. These elements of the Khitan/Jin system were adopted by the Mongols: armoured cavalry operating as distinct force on the battlefield, the 100 and 1,000 system, with larger units (tümen) in principle composed of 10 minghan of 1,000 each but in practice with fewer minghan. Adopted armoured cavalry on a large scale had to wait until the submission of the Uighurs (which they did quite willingly in 1209, in return for Mongol military support for their revolt against Khitan rule). Uighur industry and metal-working armoured Mongol forces, and the Mongols adopted Uighur administrative practices and the Uighur script.

This re-organisation of army units was politically important for Genghis - it enabled him to replace clan/tribal commanders of clan/tribal units with his own commanders (typically his relatives or his followers chosen for the combination of military ability and loyalty). Apart from providing leadership loyal to Genghis, and more stability in army organisation, it also allowed a higher level of discipline to be demanded from the soldiers.

Even during their struggle to control Mongolia, the Mongols depended on allies and subject peoples in their army. As they expanded outside the steppe, their allies and subjects began to include sedentary peoples, and a broad mix of military skills and technologies. This was the main route by which the Mongol armies acquired infantry forces, sophisticated siege techniques (though as early as their war against Xia (Xi Xia/the Tanguts) in 1209, they were performing large scale siege operations, such as building dams in an attempt to flood the Xia capital (unsuccessfully, it turned out, since the dam failed and it was the Mongols who were flooded)), and naval forces. Thus, as allies left them and subject people became independent, Mongol military capability returned closer to its roots - the Northern Yuan (as the Mongols were called after the expulsion of the Yuan from China) had a typical steppe army once more, no more capable of feats such as the 5-year Siege of Xiangyang that broke the Song defences along the Yangtze and the following gunpowder-driven push down the Yangtze valley under Kubilai than the Mongols before Genghis had been. However, they did retain many features of their old system, such as armoured cavalry, and technologies such as firearms - they continued to use handguns and light artillery in their wars against the Ming (but could not match the Ming in quantity of gunpowder weapons).