Mamai was a khan's son-in-law and a commander of armies, but not a descendant of Genghis Khan, so he could not take the title himself. Instead he seated a prince of the line on the throne and governed in his name. Through the Great Troubles of the 1360s and 1370s he effectively ruled the half of the Horde west of the Volga.
Legitimacy was his weak point. The Rus' princes who paid tribute began asking who the real khan was and hedging their obedience, and in 1378 his troops were beaten by a Moscow army at the Vozha River. Needing to restore his authority, he gathered a large army and marched north in 1380.
On Kulikovo Field his army broke under the surprise charge of the reserve cavalry Dmitry had hidden in the woods. He fled and tried to rebuild, but Tokhtamysh, a true Genghisid, pushed him aside almost at once. Many of his own men went over without fighting, a plain demonstration of the limits of ruling without the bloodline.