In 1241–1242, the kingdom suffered a major blow in the wake of the Mongol invasion of Europe. After Hungary was invaded by the Mongols in 1241, the Hungarian army was defeated disastrously at the Battle of Mohi. King Béla IV fled the battlefield and then the country after the Mongols pursued him to its borders. Before the Mongols retreated, a large part of the population (20-50%) died.^[22] In the plains, between 50 and 80% of the settlements were destroyed.^[23] Only castles, strongly fortified cities and abbeys could withstand the assault, as the Mongols had no time for long sieges—their goal was to move west as soon as possible. The siege engines and the Chinese and Persian engineers that operated them for the Mongols had been left in the conquered lands of Kyivan Rus'.^[24] The devastation caused by the Mongol invasions later led to the invitation of settlers from other parts of Europe, especially from Germany.

During the Mongols' campaign against Kievan Rus, some 40,000 Cumans, members of a nomadic tribe of pagan Kipchaks, were driven west of the Carpathian Mountains.^[25] There, the Cumans appealed to King Béla IV for protection.^[26] The Iranian Jassic people came to Hungary together with the Cumans after they were defeated by the Mongols. Cumans constituted perhaps up to 7–8% of the population of Hungary in the second half of the 13th century.^[27] Over the centuries they were fully assimilated into the Hungarian population, and their language disappeared, but they preserved their identity and their regional autonomy until 1876.^[28]

As a consequence of the Mongol invasions, King Béla ordered the construction of hundreds of stone castles and fortifications to help defend against a possible second Mongol invasion. The Mongols did indeed return to Hungary in 1286, but the newly built stone-castle systems and new military tactics involving a higher proportion of heavily armed knights stopped them. The invading Mongol force was defeated near Pest by the royal army of King Ladislaus IV. Later invasions were also repelled handily. The castles built by Béla IV proved to be very useful at a later time in the long struggle against the Ottoman Empire. However, the cost of building them indebted the Hungarian king to the major feudal landlords, so that the royal power reclaimed by Béla IV after his father Andrew II significantly weakened it was once again dispersed among the lesser nobility.