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1840 (MDCCCXL) was a leap year starting on Wednesday (dominical letter ED) of the Gregorian calendar and a leap year starting on Monday (dominical letter GF) of the Julian calendar, the 1840th year of the Common Era (CE) and Anno Domini (AD) designations, the 840th year of the 2nd millennium, the 40th year of the 19th century, and the 1st year of the 1840s decade. As of the start of 1840, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
- January 3 – One of the predecessor papers of the Herald Sun of Melbourne, Australia, The Port Phillip Herald, is founded.
- January 10 – Uniform Penny Post is introduced in the United Kingdom.
- January 13 – The steamship Lexington burns and sinks in icy waters, four miles off the coast of Long Island; 139 die, only four survive.
- January 19 – Captain Charles Wilkes' United States Exploring Expedition sights what becomes known as Wilkes Land in the southeast quadrant of Antarctica, claiming it for the United States and providing evidence that Antarctica is a complete continent.
- January 21 – Jules Dumont d'Urville discovers Adélie Land in Antarctica, claiming it for France.
- January 22 – British colonists reach New Zealand, officially founding Wellington.
- February – The Rhodes blood libel is made against the Jews of Rhodes.
- February 5 – The murder of a Capuchin friar and his Greek servant leads to the Damascus affair, a highly publicized case of blood libel against the Jews of Damascus.
- February 6 – The Treaty of Waitangi, granting British sovereignty in New Zealand, is signed.
- February 10 – Queen Victoria of the United Kingdom marries her cousin Prince Albert of Saxe-Coburg and Gotha.
- February 11 – Gaetano Donizetti's opera La fille du régiment premieres in Paris.
- March 1
- March 9 – The Wilmington and Raleigh Railroad is completed from Wilmington, North Carolina to Weldon, North Carolina. At 161.5 miles (260 km), it is the world's longest railroad.
- July 4 – The Cunard Line's 700-ton wooden paddlewheel steamer RMS Britannia departs from Liverpool, bound for Halifax, Nova Scotia on the first steam transatlantic passenger mail service.
- July 15 – The Austrian Empire, the United Kingdom, the Kingdom of Prussia, and the Russian Empire sign the Convention of London with the Sublime Porte, ruler of the Ottoman Empire.
- July 21 – August Borsig's steam locomotive, the first built in Germany, competes against a Stephenson-built locomotive on the Berlin–Jüterbog railroad; the Borsig locomotive wins by 10 minutes.
- July 23
- August 10 – Fortsas hoax: A number of book collectors gather in Binche, Belgium, to attend a non-existent book auction of the late "Count of Fortsas".
- September 10 – Ottoman and British troops bombard Beirut and land troops on the coast to pressure Egyptian Muhammad Ali to retreat from the country.
- September 16 – Joseph Strutt hands over the deeds and papers concerning the Derby Arboretum, which is to become England's first public park.
- September 30 – The frigate Belle-Poule arrives in Cherbourg, bringing back the remains of Napoléon from Saint Helena to France. He is buried in Les Invalides.
The frigate Belle-Poule brings back the remains of Napoléon to France.
- January 1 – Dugald Drummond, British railway engineer (d. 1912)
- January 3 – Father Damien, Belgian missionary priest (d. 1889)
- January 18 – Alfred Percy Sinnett, British writer (d. 1921)
- January 21 – Sophia Jex-Blake, English physician (d. 1912)
- January 22 – Ernest Roland Wilberforce, English bishop (d. 1907)
- January 23 – Ernst Abbe, German physicist (d. 1905)
- January 26 – John Clayton Adams, British landscape artist (d. 1906)
- February 4 – Hiram Stevens Maxim, American firearms inventor (d. 1916)
- February 5 – John Boyd Dunlop, Scottish inventor (d. 1921)
- February 9 – William T. Sampson, American admiral (d. 1902)
- February 15 – Titu Maiorescu, Romanian Prime Minister (d. 1917)
- February 21 – Murad V, Ottoman Sultan (d. 1904)
- February 22 – August Bebel, German politician (d. 1913)
- February 23 – Carl Menger, Austrian economist (d. 1921)
- February 29 – John Philip Holland, Irish inventor (d. 1914)
- March 12 – Masataka Kawase, a.k.a. Kogorō Ishikawa, Japanese political activist and diplomat (d. 1919)
- March 28 – Emin Pasha, German doctor and African administrator (d. 1892)
- March 31 – Benjamin Baker, English civil engineer (d. 1907)
- April 9 – Praskovya Uvarova, Russian archaeologist (d. 1924)
- April 2 – Émile Zola, French writer (d. 1902)
- April 22 – Odilon Redon, French painter (d. 1916)
- April 27 – Edward Whymper, English mountaineer (d. 1911)
- May 7 – Pyotr Ilyich Tchaikovsky, Russian composer (d. 1893)
- May 13 – Alphonse Daudet, French writer (d. 1897)
- June 2
- June 7 – Charlotte of Belgium, Princess of Belgium and Empress of Mexico (d. 1927)
- June 10 – Theodor Philipsen, Danish painter (d. 1920)
- June 13 – Augusta Lundin, Swedish fashion designer (d. 1919)
- June 14 – William F. Nast, American attaché, railroad executive and inventor, and father of Condé Nast (d. 1893)
- June 21 – Edward Stanley Gibbons, English philatelist and founder of Stanley Gibbons Ltd (d. 1913)
- August 4 – Richard von Krafft-Ebing, German sexologist (d. 1902)
- August 25 – George C. Magoun, American railroad executive (d. 1893)
- September 22 – D. M. Canright, American Seventh-day Adventist minister and author, later one of the church's severest critics (d. 1919)
- September 27 – Alfred Thayer Mahan, United States Navy admiral and American geostrategist and historian (d. 1914)
- October 9 – Simeon Solomon, British artist (d. 1905)
- October 12 – Helena Modjeska, Polish stage actress (d. 1909)
- October 16 – Kuroda Kiyotaka, Prime Minister of Japan (d. 1900)
- November 7 – H. G. Haugan, Norwegian-born American railroad and banking executive (d. 1921)
- November 12 – Auguste Rodin, French sculptor (d. 1917)
- November 14 – Claude Monet, French painter (d. 1926)
- November 21 – Victoria, Princess Royal of the United Kingdom and Empress of Germany (d. 1901)
- November 29 – Rhoda Broughton, Welsh writer (d. 1920)
- December 17 – Nozu Michitsura, Japanese general (d. 1908)
- January 6 – Fanny Burney, English novelist (b. 1752)
- January 22 – Johann Friedrich Blumenbach, German anthropologist (b. 1752)
- February 13 – Nicolas Joseph Maison, French marshal and Minister of War (b. 1770)
- March 11 – George Wolf, American politician (b. 1777)
- April 25 – Siméon Denis Poisson, French mathematician, geometer, and physicist (b. 1781)
- May 1 – Joseph Williamson, philanthropist and builder of the Williamson Tunnels (b. 1769)
- May 6 – Demetrius Augustine Gallitzin, Russian aristocrat and priest (b. 1770)
- May 7 – Caspar David Friedrich, German artist (b. 1774)
- May 14 – Carl Ludvig Engel, German-Finnish architect (b. 1778)
- May 26 – Sidney Smith, British admiral (b. 1764)
- May 25 – Louisa Capper, English writer, philosopher and poet (b. 1776)
- May 27 – Niccolò Paganini, Italian violinist and composer (b. 1782)
- June 7 – King Frederick William III of Prussia (b. 1770)