- January – The World Tomorrow pacifist magazine begins publication.
- January 8 – President Woodrow Wilson delivers his Fourteen Points speech.
- February 21 – The last Carolina parakeet (the last breed of parrot native to the eastern U.S.), a male named “Incas”, dies at Cincinnati Zoo.
- March – The Liberator socialist magazine begins publication.
- March 4 – A soldier at Camp Fuston, Kansas falls sick with the first confirmed case of the Spanish flu.
- March 19 – The U.S. Congress establishes time zones and approves daylight saving time (DST goes into effect on March 31).
- April 21 – The 6.7 Mw San Jacinto earthquake shook southern California with a maximum Mercalli intensity of IX (Severe), causing $200,000 in damage, one death, and several injuries.
- May 2 – General Motors acquires the Chevrolet Motor Company of Delaware.
- May 15 – The United States Post Office Department (later renamed the United States Postal Service) begins the first regular airmail service in the world (between New York City, Philadelphia and Washington, DC).
- May 16 – The Sedition Act of 1918 is approved by the U.S. Congress.
- May 20 – The small town of Codell, Kansas is hit for the third year in a row by a tornado. Coincidentally, all three tornadoes hit on May 20, 1916, 1917, and 1918 respectively.
- June 8 – The total solar eclipse of June 8, 1918 crossed the United States from Washington State to Florida.
- June 22 – Suspects in the Chicago Restaurant Poisonings are arrested, and more than 100 waiters are taken into custody, for poisoning restaurant customers with a lethal powder called Mickey Finn.
- July 9 – Great train wreck of 1918: In Nashville, Tennessee, an inbound local train collides with an outbound express, killing 101.
- August – A deadly second wave of the Spanish flu starts in France, Sierra Leone, and the United States.
- September 11 – The Boston Red Sox defeat the Chicago Cubs for the 1918 World Series championship, their last World Series win until 2004.
- October 8 – World War I: In the Forest of Argonne in France, U.S. Corporal Alvin C. York almost single-handedly kills 25 German soldiers and captures 132.
- October 11 – The 7.1 Mw Puerto Rico earthquake shakes the island with a maximum Mercalli intensity of IX (Violent), killing 76–116 people. A destructive tsunami contributed to the damage and loss of life.
- October 12 – 1918 Cloquet Fire: The city of Cloquet, Minnesota and nearby areas are destroyed in a fire, killing 453.
- October 25 – The SS Princess Sophia sinks on Vanderbilt Reef near Juneau, Alaska; 353 people die in the greatest maritime disaster in the Pacific Northwest.
- November 1 – Malbone Street Wreck: The worst rapid transit accident in world history occurs under the intersection of Malbone Street and Flatbush Avenue, in Brooklyn, New York City, with at least 93 dead.
- November 11 – World War I ends.
- December 4 – President of the U.S. Woodrow Wilson sails for the Paris Peace Conference, becoming the first U.S. president to travel to Europe while in office.
- December 19 – Ripley’s Believe It or Not! first appears as a cartoon under the title Champs and Chumps in The New York Globe.