The 1240s was a decade of the Julian Calendar which began on January 1, 1240, and ended on December 31, 1249.
- March 18 – Battle of Chmielnik (Mongol invasion of Poland): The Mongols overwhelm the feudal Polish armies of Sandomierz and Kraków provinces, and plunder the abandoned city of Kraków.
- April 9 – Battle of Legnica: The Mongols, under the command of Baidar, Kadan and Orda Khan, defeat the feudal Polish nobility, including the Knights Templar.
- April 11 – Battle of Mohi: Batu Khan and Subutai defeat Béla IV of Hungary. The battle is the last major event in the Mongol Invasion of Europe.
- April 27 – Battle of Sajo: The Mongols defeat Bela IV of Hungary.
- May – Battle of Giglio: an Imperial fleet defeats a Genoan fleet in the Tyrrhenian Sea.
- May 10 – Battle of Cameirge in Ulster: The Milesian Irish septs of the Ó Dónaills from Donegal, the Ó Néills from Armagh and the Ó Dochartaighs of Connacht defeat the last Tuatha Dé Danann sept, the Meic Lochlainn of Tír Eoghain and Inishowen under Domhnall mac Muirchertaigh Mac Lochlainn. From now on the Kings of Tír Eoghain will all be of the Ó Néill dynasty, Brian Ua Néill becoming sole ruler.
- Early northern summer – A succession crisis or other priorities results in the Mongols withdrawing behind their river barrier into the Ukraine and the Russias, leaving Central Asian and far Eastern Europe peoples tributary to the Khanates, but leaving Poland and Hungary to begin recovery and reorganization.
- August 29 – After Henry III of England's invasion of Wales, the Treaty of Gwerneigron is signed by him and Dafydd ap Llywelyn, curbing the latter's authority and denying him royal title.
- September 23 – Snorri Sturluson, Icelandic saga writer, is murdered by Gissur Þorvaldsson, an emissary of King Haakon IV of Norway.
- October 25 – Pope Celestine IV succeeds Pope Gregory IX, as the 179th pope.
- Emperor Lizong of Song China accepts the Neo-Confucian teachings of the late Zhu Xi, including his commentary on the Four Books. This will have an impact upon the philosophical schools of surrounding countries as well, including Korea, Japan, and Vietnam.
- Livonian Crusade: The Estonian rebellion of 1237 is suppressed on Saaremaa Island, by the Livonian Order.
- The diocese of Warmia, Poland is created.
- July 2 – A document issued by king Béla IV of Hungary, granting territories to the Knights Hospitaller in the Banate of Severin and Cumania, makes an early mention of Litovoi and other Vlach/Romanian local rulers, in Wallachia and Transylvania.
- December 1 – A rebellion arises among the Muslim subjects of the Crown of Aragon, in the region of Valencia. As a punishment, the king issues an order of expulsion of the Muslims from his realm, leading numerous people into exile in Andalusia and North Africa, in the subsequent year.
- Shams Tabrizi disappears, resulting in Jalal Uddin Rumi writing 30,000 verses of poetry about his disappearance.
- Romford, London, England is chartered as a market town.
- The future Bethlem Royal Hospital, Bedlam, is founded in London.
- The Thuringian War of Succession begins.
- Qin Jiushao publishes the original form of the Chinese remainder theorem.
- Egypt takes control of Jerusalem from the Kharezmians.
- Nijmegen becomes part of Gelderland.
- Afonso III succeeds Sancho II, as King of Portugal.
- Song Ci publishes the Collected Cases of Injustice Rectified, a book considered to be the first monographic work on forensic medicine.
- Jean Mouflet makes an agreement with the abbot of Saint-Pierre-le-Vif in the Senonais region in France: in return for an annual payment, the monastery will recognize Jean as a "citizen of Sens". He is a leather merchant, with a leather shop that he leases for the rent of 50 shillings a year. The agreement is witnessed by Jean's wife, Douce, daughter of a wealthy and prominent citizen of Sens, Felis Charpentier.