|Covid-19|January 20 – Emperor Theophilos dies of dysentery at Constantinople, after a 12-year reign, in which he expended much effort defending the eastern frontier, against the invading Muslim Arabs. Theophilos is succeeded by his 2-year-old son Michael III, with his mother Theodora, who becomes regent and the 'temporary' sole ruler of the Byzantine Empire.
February 14 – Oaths of Strasbourg: King Louis the German, ruler of East Francia, and his half-brother Charles the Bald, ruler of West Francia, meet with their armies at Strasbourg. They agree to swear allegiance (recorded in vernacular languages) to each other, and to support each other against their brother Lothair I (nominal emperor of all the Frankish kingdoms and the Holy Roman Empire).
March 20 – King Alfonso II of Asturias (Northern Spain) dies after a 50-year reign, in which he undertook numerous campaigns against the Muslim armies of the Umayyad Emirate of Córdoba, and allied himself with the late Charlemagne. The childless Alfonso chooses Ramiro I, son of former king Bermudo I, as his successor.
Uurad of the Picts dies after a 3-year reign, and is succeeded by his son Bridei VI, who contests his power with rival groups, led by Bruide son of Fokel and Kenneth MacAlpin.
January 5 – Caliph Al-Mu'tasim dies at Samarra (modern Iraq), after a 8-year reign. He is succeeded by his son Al-Wathiq, as ruler of the Abbasid Caliphate.
Al-Mundhir, Muslim emir (approximate date)
Al-Muwaffaq, Muslim prince and regent (d. 891)
Li Hanzhi, Chinese warlord (d. 899)
Pietro I Candiano, doge of Venice (approximate date)
Yang Fuguang, Chinese general (d. 883)
January 5 – Al-Mu'tasim, Muslim caliph (b. 796)
January 20 – Theophilus, Byzantine emperor (b. 813)
March 9 – Humbert, bishop of Würzburg
March 16 – Xiao Mian, chancellor of the Tang Dynasty
August 24 – Saga, emperor of Japan (b. 786)
October 22 – Abo, Japanese prince (b. 792)
Alfonso II, king of Asturias (b. 759)
Bernard of Vienne, Frankish bishop (b. 778)
Dúngal mac Fergaile, king of Osraige (Ireland)
Li Cheng, chancellor of the Tang Dynasty
Liu Yuxi, Chinese poet and philosopher (b. 772)
Sugawara no Kiyotomo, Japanese nobleman (b. 770)
Uurad, king of the Picts (approximate date)
We Gyaltore Taknye, Tibetan nobleman
Zheng Tan, chancellor of the Tang Dynasty
Year 842 (DCCCXLII) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar.