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1742 (MDCCXLII) was a common year starting on Monday of the Gregorian calendar and a common year starting on Friday of the Julian calendar, the 1742nd year of the Common Era (CE) and Anno Domini (AD) designations, the 742nd year of the 2nd millennium, the 42nd year of the 18th century, and the 3rd year of the 1740s decade. As of the start of 1742, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.
- January – The House of Commons of Great Britain votes on the alleged rigging of the Chippenham by-election. It becomes a motion of no confidence, which leads to the resignation of Robert Walpole.
- January 9 – Robert Walpole is made Earl of Orford, and resigns as First Lord of the Treasury and Chancellor of the Exchequer, effectively ending his period as Prime Minister of Great Britain. On his formally relinquishing office five days later, he will have served 20 years and 314 days as Prime Minister, the longest single term ever, and also longer than the accumulated terms of any other British Prime Minister.
- January 24 – Charles VII Albert becomes Holy Roman Emperor.
- February 12 – John Carteret, 2nd Lord Carteret becomes Secretary of State for the Northern Department in Great Britain.
- February 16 – Spencer Compton, Earl of Wilmington, becomes Prime Minister of Great Britain.
- April 13 – George Frideric Handel's oratorio The Messiah is first performed in Dublin, Ireland.
- May 17 – Frederick the Great's army defeats the Austrians in Chotusitz.
- May 24 – War of the Austrian Succession – Battle of Sahay: French forces defeat the Austrians.
- June 11 – Peace of Breslau: Austria cedes Silesia to Prussia.
- Daniel le Pelley succeeds Nicolas le Pelley, as Seigneur of Sark.
- The Kingdom of Prussia captures Jihlava.
- Christian Goldbach formulates Goldbach's conjecture.
- Colin Maclaurin publishes his Treatise on Fluxions.
- Anders Celsius proposes the Celsius temperature scale (see 1741).
- James Bradley succeeds Edmond Halley, as Astronomer Royal.
- The University of Erlangen is founded.
- The Lopukhina Conspiracy arises at the Russian court.
- Molde, Norway, becomes a city.
- Eisenach, Germany builds its Stadtschloss (city castle).
- In Peru, Juan Santos takes the name Atahualpa II, and begins an ill-fated rebellion against the Spanish rule.
- The Afghan tribes unite as a monarchy.
- Henry Fielding publishes Joseph Andrews.
- Charles Jervas's English translation of Don Quixote is published posthumously. Through a printer's error, the translator's name is printed as Charles Jarvis, leading the book to forever be known as the Jarvis translation. It is acclaimed as the most faithful English rendering of the novel made up to that time.
- Spain completes the construction of Fort Matanzas in the Matanzas Inlet, approximately 15 miles (24 km) south of St. Augustine, Florida.
- Rome decrees that Roman ceremonial practice in Latin (not in Chinese) is to be the law for Chinese missions.
- March 14 – Agha Mohammad Khan Qajar, Iranian king (d. 1797)
- March 12 – Aletta Haniel, German business person (d. 1815)
- April 28 – Henry Dundas, British statesman (d. 1811)
- May 6 – Jean Senebier, Swiss pastor, botanist (d. 1809)
- June 25 – Johann Schweighäuser, German classical scholar (d. 1830)
- June 26 – Arthur Middleton, American politician (d. 1787)
- June 28 – William Hooper, American statesman (d. 1790)
- July 21 – John Cleves Symmes, American statesman (d. 1814)
- July 27 – Nathanael Greene, American general (d. 1786)
- August 14 – Pope Pius VII (b. Barnaba Niccolò Maria Luigi Chiaramonti), Italian Benedictine (d. 1823)
- January 14 – Edmond Halley, English astronomer (b. 1656)
- February 22 – Charles Rivington, English publisher (b. 1688)
- March 23 – Jean-Baptiste Dubos, French author (b. 1670)
- April 2 – James Douglas, Scottish physician, anatomist (b. 1675)
- April 15 – Samuel Shute, Governor of Massachusetts Bay and New Hampshire (b. 1662)
- April 17 – Arvid Horn, Swedish statesman (b. 1664)
- May 13 – Ludwig IX, Landgrave of Hesse-Darmstadt (b. 1719)
- May 21 – Lars Roberg, Swedish physician (b. 1664)
- May 26 – Pylyp Orlyk, Ukrainian Zaporozhian Cossack starshina, diplomat (b. 1672)
- June 18 – John Aislabie, British politician (b. 1670)
- June 27 – Nathan Bailey, English philologist, lexicographer
- July 1 – Bohuslav Matěj Černohorský, Czech composer (b. 1684)
- July 4 – Guido Grandi, Italian mathematician (b. 1671)
- July 9 – John Oldmixon, English historian (b. 1673)
- July 12 – Evaristo Abaco, Italian composer (b. 1675)
- July 14 – Richard Bentley, English scholar and critic (b. 1662)
- July 19 – William Somervile, English poet (b. 1675)
- July 23 – Susanna Wesley, English mother of the Wesley brothers (b. 1669)
- July 30 – Nicholas Roosevelt (1658–1742), Dutch-American politician (b. 1658)
- August 14 – Maria van Lommen, Dutch gold- and silversmith and guild member (b. 1688)
- August 25 – Carlos Seixas, Portuguese composer (b. 1704)
- September 18 – Vincenzo Ludovico Gotti, Italian Catholic cardinal (b. 1664)
- September 22 – Frederic Louis Norden, Danish explorer (b. 1708)
- September 27 – Hugh Boulter, Irish Archbishop of Armagh (b. 1672)
- September 28 – Jean Baptiste Massillon, French bishop (b. 1663)
- November 12 – Friedrich Hoffmann, German physician, chemist (b. 1660)
- November 20 – Melchior de Polignac, French diplomat (b. 1661)
- November 24 – Andrew Bradford, American publisher (b. 1686)
- December 18 – William Fairfield, Massachusetts Speaker of the House of Deputies (b. 1662)
- December 31 – Karl III Philip, Elector Palatine (b. 1661)