February 25, 1866 – Miners in Calaveras County, California, discover what is now called the Calaveras Skull, human remains that supposedly indicated that man, mastodons, and elephants had co-existed.

April 6, 1866 – The Grand Army of the Republic, an American patriotic organization composed of Union veterans of the American Civil War, is founded. It lasts until 1956.

April 10, 1866 – The American Society for the Prevention of Cruelty to Animals (ASPCA) is founded in New York City by Henry Bergh.

May 5, 1866 – Memorial Day first celebrated in United States at Waterloo, New York.

May 31, 1866 – In the Fenian Invasion of Canada, John O'Neill leads 850 Fenian raiders across the Niagara River at Buffalo, New York/Fort Erie, Ontario, as part of an effort to free Ireland from the United Kingdom. Canadian militia and British regulars repulse the invaders in over the next three days, at a cost of 9 dead and 38 wounded to the Fenian’s 19 dead and about 17 wounded.

June 24, 1866 – Battle of Custoza: an Austrian army defeats the Italian army during the Austro-Prussian War.

July 24, 1866 – Reconstruction: Tennessee becomes the first U.S. State to be readmitted to the Union following the American Civil War.

July 27, 1866 – The first permanent transatlantic telegraph cable is successfully completed, stretching from Valentia Island, Ireland, to Heart’s Content, Newfoundland.

September 22, 1866 – Battle of Curupaity in the War of the Triple Alliance.

October 19, 1866 – Venice - Annexion of Veneto and Mantua to Italy - At Hotel Europa, Austria hands over Veneto to France, which hands it immediately over to Italy.

January 2, 1866 – Prof. Gilbert Murray, Australian classical scholar (d. 1957)

January 13, 1866 – Vasily Kalinnikov, Russian composer (d. 1901)

January 15, 1866 – Nathan Söderblom, Swedish archbishop, recipient of the Nobel Peace Prize (d. 1931)

February 13, 1866 – Lev Shestov, Russian philosopher (d. 1938)

February 24, 1866 – Pyotr Nikolaevich Lebedev, Russian physicist (d. 1912)

June 4, 1866 – Miina Sillanpää, Finnish politician (d. 1952)

September 7, 1866 – Tristan Bernard, French playwright and novelist (d. 1947)

October 29, 1866 – Antonio Luna, Filipino revolutionary general (d. 1899)

November 8, 1866 – Herbert Austin, English automobile pioneer (d. 1941)

December 10, 1866 – Louis Bolk, a Dutch anatomist (d. 1930)