Events of 1840

January - June

• January 3 - One of the predecessor papers of the Herald Sun of Melbourne, Australia, The Port Phillip Herald, is founded.
• January 10 - Uniform penny postage introduced in the UK.
• January 13 - The steamship Lexington burns and sinks in icy waters, four miles off the coast of Long Island: 139 died, only 4 survived.
• January 19 - Captain Charles Wilkes circumnavigates Antarctica, claiming what became known as Wilkes Land for the United States.
• January 20
  • Dumont D'Urville discovers Adélie Land, Antarctica.
  • The first public school is established in Rockingham County
• January 22 - British colonists reach New Zealand. Official founding date of Wellington.
• February - Rhodes blood libel 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 - Treaty of Waitangi, document granting British sovereignty in New Zealand, is signed.
• February 10 - Queen Victoria of the United Kingdom marries Prince Albert of Saxe Coburg-Gotha.
• February 11 - Gaetano Donizetti's opera La Fille du Regiment premieres in Paris.
• March 1
  • William Hobson, first Governor of New Zealand, suffer a stroke.
  • Adolphe Thiers becomes prime minister of France.
• April 15 - King's College Hospital opens in Portugal Street, London.
• May 1 - Britain issues the Penny Black, world's first postage stamp.
• May 6 - The Penny Black, world's first postage stamp becomes valid for the pre-payment of postage.
• May 7 - The Great Natchez Tornado: A massive tornado strikes Natchez, Mississippi during the early afternoon hours. Before it was over, 317 people had lost their lives and 109 were injured. It is the second deadliest tornado in U.S. history.
• May 21 - New Zealand is declared a British colony.

July - December