- Apr 2 Skirmish at Crump's Hill (Piney Woods), Louisiana
- Apr 2 Skirmish at Spoonville, Antoine, Arkansas
- Apr 3 Skirmish at Okolona, Arkansas
- Apr 4 US Civil War: Skirmish at Elkin's Ford (Little Missouri River), Arkansas
- Apr 8 Battle of Mansfield, Louisiana; Federals routed by General Richard Taylor
- Apr 9 Battle of Pleasant Hill, Louisiana, Confederate assault repulsed by Union side with high cost of estimated 3,100 causalities
- Apr 9 Union surgeon Mary Edwards Walker is captured by Confederate troops and arrested as a spy during US Civil War
Maximilian I
Apr 10 Austrian Archduke Maximilian becomes Maximilian I, Emperor of Mexico
- Apr 12 Battle of Blair's Landing, Louisiana
Battle of Fort Pillow
Apr 12 Confederate General Nathan Bedford Forrest captures Fort Pillow, Tennessee
- Apr 15 General Steeles' Union troops occupies Camden, Arkansas
- Apr 17 Battle of Plymouth, North Carolina
- Apr 17 Bread revolt in Savannah, Georgia
- Apr 17 Grant suspends prisoner-of-war exchanges
- Apr 18 Battle of Dybbøl: Denmark is defeated by Prussian and Austrian forces storming Dybbol fort in the deciding battle of the Second Schleswig War
- Apr 18 Battle of Poison Springs, Arkansas (Camden Expedition)
- Apr 22 US mints 2 cent coin (1st appearance of "In God We Trust")
- Apr 23 Battle of Cane River, Louisiana (Red River Expedition, Monett's Ferry)
- Apr 25 Battle of Marks' Mill, Arkansas (Camden Expedition)
- Apr 29 -30] Skirmish at Jenkins' Ferry, Arkansas
Battle of Gate Pā
Apr 29 Battle of Gate Pā (Pukehinahina): 1,700 British troops suffer their worst defeat of the New Zealand Wars at the hands of 230 entrenched Māori warriors in Tauranga
- Apr 29 The Theta Xi fraternity is founded at Rensselaer Polytechnic Institute in Troy, New York
- Apr 30 Battle of Jenkin's Ferry, Arkansas; Confederate General William Reid Scurry is killed
- Apr 30 New York becomes 1st state to charge a hunting license fee
Admirals
Aviators
Gangsters
Generals
Industrialists
Suffragists