- Oct 1 Cyclone strikes Calcutta: 70,000 killed
- Oct 1 John Summerfield Staples is paid $500 as a substitute for US President Abraham Lincoln
- Oct 2 Battle of Saltville, Virginia
- Oct 4 National black convention meets in Syracuse, New York
- Oct 4 New Orleans Tribune, 1st black daily newspaper, forms
- Oct 5 Battle of Allatoona, 1/3 of Union troops die repulsing South
- Oct 5 Most of Calcutta destroyed by cyclone, approx 60,000 die
- Oct 7 -Oct 13th) Battle of Darbytown Road, Virginia
- Oct 7 Naval Engagement at Bahia Harbor, Brazil - CSS Florida vs USS Wachusett
Victory in Battle
Oct 9 Battle of Tom's Brook; Confederate cavalry that harassed Sheridan's campaign is beaten by General George A. Custer & Merrit's cavalry divisions
George Armstrong Custer
Event of Interest
Oct 12 War of the Triple Alliance begins with Brazilian troops under the command of Gen. João Propício Mena Barreto invading Uruguay
Francisco Solano López
John Singleton Mosby
- Oct 13 Maryland voters adopt new constitution, including abolition of slavery
- Oct 15 Confederate troops occupy Glasgow, Missouri
- Oct 19 Approx 25 Confederates make surprise attack on St Albans, Vermont
- Oct 19 Battle of Cedar Creek, Virginia, Union beats back Confederate attackers
- Oct 19 US Brigadier General Emory Upton (25) promoted to Major General
Event of Interest
Oct 20 US President Abraham Lincoln formally establishes Thanksgiving as a national holiday
Abraham Lincoln
- Oct 21 Battle of Westport, MO (Kansas City)
- Oct 23 Battle of Westport, Missouri: Union General Samuel R Curtis defeats Confederate General Stirling Price
- Oct 25 Battle of Marais Des Cygnes River, Kansas (Mine Creek)
- Oct 26 Union troops ambush & kill 'Bloody' Bill Anderson near Albany, Missouri
- Oct 27 Battle of Boydton Plank Road, VA (Burgess' Mill, Southside Railroad)
- Oct 27 Second Battle of Fair Oaks, Virginia near Richmond
- Oct 27 Confederate ship Albemarle torpedoed and sunk by a spar torpedo mounted on a steam launch
- Oct 28 Second Battle of Newtonia (American Civil War), Newton County, Missouri
- Oct 28 Second Battle at Fair Oaks, Virginia, ends with 1554 casualties
- Oct 28 Battle of Wauhatchie, Tennessee
- Oct 29 Greek parliament accept new Constitution
- Oct 30 Helena, Montana's capital, founded
- Oct 30 Treaty of Vienna signed to end the Second Schleswig War, Denmark losses control of Schleswig to Prussia and Holstein to Austria
- Oct 31 Nevada admitted as 36th state of the Union