Year 1140 (MCXL) was a leap year starting on Monday of the Julian calendar.

• Spring – King Fulk of Jerusalem confronts Imad al-Din Zengi, Seljuk ruler (atabeg) of Mosul, near Dara'a in southern Syria.^[1] Turkish forces under Mu'in al-Din (supported by the Crusaders) besiege Banias.

• Spring – King Conrad III enfeoffs Henry II (Jasomirgott), a member of the House of Babenberg, with the County Palatine of the Rhine (belonging to the Holy Roman Empire).
• Summer – King Roger II promulgates the Assizes of Ariano (a series of laws to rule the Norman Kingdom of Sicily) after the pacification of southern Italy.
• December 21 – Siege of Weinsberg: Conrad III captures the castle at Weinsberg during the civil war between the Staufers and the Welfs in Germany.

• Summer – King Stephen appoints Geoffrey de Mandeville as Earl of Essex for his support during the civil war against Matilda (Stephen's cousin).
• The town of Lanark in Scotland is made a Royal Burgh by King David I, giving it merchant privileges relating to government and taxation.

• August 21 – Jin–Song Wars – Battle of Yancheng: Chinese forces under the command of Yue Fei defeat a numerically superior Jin army led by Wuzhu.

• June 3 – Pierre Abelard, a French theologian, is condemned for heresy by the Council of Sens. He sets out for Rome to present his defense to Pope Innocent II.
• September 8 – Sephardi Jewish philosopher Judah Halevi, having completed the Kuzari, arrives in Alexandria on a pilgrimage to Palestine.
• The first Cistercian monastery in Spain is founded in Fitero.^[2] The order enjoys a rapid expansion in the region in the following 15 years.

• Gratian, an Italian monk and canon lawyer, founds the science of Canon law with the publication of the Decretum Gratiani (approximate date).

• May 28 – Xin Qiji, Chinese military leader (d. 1207)
• Adela of Champagne, queen of France (d. 1206)
• Alan Fitz Walter, Scottish High Steward (d. 1204)
• Cadfan ap Cadwaladr, Welsh nobleman (d. 1215)
• Davyd Rostislavich, Kievan Grand Prince (d. 1197)
• Domhnall Caomhánach, king of Leinster (d. 1175)
• Eliezer ben Joel HaLevi, German rabbi (d. 1225)
• Fujiwara no Tashi, Japanese empress (d. 1202)
• Gerard de Ridefort, Flemish Grand Master (d. 1189)
• Hedwig, margravine of Meissen (approximate date)
• Hugh de Paduinan, Norman nobleman (d. 1189)
• John I, archbishop of Trier (approximate date)
• John I, Norman nobleman (approximate date)
• John of Ford, English Cistercian abbot (d. 1224)
• Manfred II, marquess of Saluzzo (approximate date)
• Minamoto no Yoshihira, Japanese nobleman (d. 1160)
• Peter Waldo, French spiritual leader (d. 1205)
• Raymond III, crusader and count of Tripoli (d. 1187)
• Raymond the Palmer, Italian pilgrim (d. 1200)
• Simon II, duke of Lorraine (approximate date)
• Sophia of Minsk, queen of Denmark (d. 1198)
• Walter Map, Welsh clergyman and writer (d. 1210)
• William FitzRalph, English High Sheriff (d. 1200)
• Yuan Cai, Chinese scholar and official (d. 1195)

• January 12 – Louis I, German nobleman
• February 6 – Thurstan, archbishop of York
• February 14
  • Leo I, prince of Armenia
  • Soběslav I, duke of Bohemia
• August 21 – Yang Zaixing, Chinese general
• August 31 – Godebold, bishop of Meissen
• September 15 – Adelaide, duchess of Bohemia
• November 16 – Wulgrin II, count of Angoulême
• Aibert, French monk and hermit (b. 1060)
• Baldwin of Rieti, Italian Benedictine abbot
• Diego Gelmírez, Galician archbishop (b. 1069)
• Gaucherius, French priest and hermit (b. 1060)
• Hugh the Chanter, English historian and writer
• Kumarapala, Indian ruler of the Pala Empire
• Lhachen Naglug, Indian ruler of Ladakh (b. 1110)
• Li Gang, Chinese Grand Chancellor (b. 1083)
• Toba Sōjō, Japanese artist-monk (b. 1053)
• Wanyan Xiyin, Chinese chief adviser