The years from 1840 to 1850 were marked by war, political changes, a gold rush in California, and many other important events in America and around the world.
- January 10: Penny postage was introduced in Britain.
- January 13: In a shocking maritime disaster, the steamship Lexington burned and sank in Long Island Sound. Only four men survived and more than 150 passengers and crew perished.
- February 10: Queen Victoria of England married Prince Albert of Saxe Coburg-Gotha.
- May 1: The first postage stamps, Britain's “Penny Black,” were issued.
- Summer/Fall: The 1840 presidential campaign was the first to prominently feature songs and slogans. William Henry Harrison won the presidency thanks to his "Log Cabin and Hard Cider" campaign, and the slogan "Tippecanoe and Tyler Too!"
- March 4: William Henry Harrison was inaugurated as president of the United States. He delivered a two-hour inaugural address in very cold weather. As a result, he caught pneumonia, from which he never recovered.
- Spring: A free black New Yorker, Solomon Northup, was lured to Washington, D.C., drugged, and kidnapped into slavery. He would tell his story in the powerful memoir "Twelve Years a Slave."
- April 4: President William Henry Harrison died after only one month in office. He was the first American president to die in office and was succeeded by Vice President John Tyler.
- Autumn: Land was purchased in Massachusetts for Brook Farm, an experimental farming community frequented by Nathaniel Hawthorne, Ralph Waldo Emerson, and other writers and thinkers of the era.
- November 9: Edward VII of England, son of Queen Victoria and Prince Albert, was born.
- January: The British retreated from Kabul, Afghanistan and were massacred by Afghan troops.
- August 29: The First Opium War ended with the Treaty of Nanking.
- November: Showman Phineas T. Barnum tracked down a child in Connecticut said to be peculiarly small. The boy, Charles Stratton, would become a show business phenomenon known as General Tom Thumb.
- Summer: "Oregon Fever" gripped America, beginning mass migration westward on the Oregon Trail.
- February 28: An accident with a cannon on US Navy warship killed two members of John Tyler's cabinet.
- May 24: The first telegram was sent from the U.S. Capitol to Baltimore. Samuel F.B. Morse wrote, “what hath God wrought.”
- August: Karl Marx and Friedrich Engels met in Paris.
- November: James Knox Polk defeated Henry Clay in the U.S. presidential election.
- January 23: The U.S. Congress established a uniform date for federal elections, naming the first Tuesday after the first Monday in November as Election Day.
- March 1: President John Tyler signed a bill annexing Texas.
- March 4: James Knox Polk was inaugurated as President of the United States.
- May: Frederick Douglass published his autobiography "Narrative of the Life of Frederick Douglass, An American Slave."
- May 20: The Franklin Expedition sets sail from Britain. All 129 men on the expedition were lost during their attempt to explore the Arctic.
- Late Summer: The Irish potato famine, which would become known as the Great Famine, started with widespread failures of the potato crop.
- February 26: American frontier scout and showman William F. “Buffalo Bill” Cody was born in Iowa.
- April 25: Mexican troops ambushed and killed a patrol of U.S. soldiers. Reports of the incident inflamed tensions between the two nations.
- April-August: Francis Parkman traveled from St. Louis, Missouri to Ft. Laramie, Wyoming, and later wrote of the experience in the classic book "The Oregon Trail."
- May 13: The U.S. Congress declared war against Mexico.
- June 14: In the Bear Flag Revolt, settlers in northern California declared independence from Mexico.
- December: The Donner Party, a party of American settlers in wagon trains, became stranded in the snow-covered Sierra Nevada Mountains in California and resorted to cannibalism to survive.
- February 22: U.S. troops commanded by General Zachary Taylor defeated a Mexican Army at the Battle of Buena Vista in the Mexican War.
- March 29: U.S. troops commanded by General Winfield Scott captured Veracruz in the Mexican War.
- June 1: Cornelius Vanderbilt, one of America's richest and most competitive men, raced a steamboat against rival Daniel Drew in the Hudson River. Many thousands of New Yorkers lined the city's docks to watch the paddle wheelers race.
- Late summer: The potato famine continued in Ireland, and the year became known as "Black '47."
- September 13-14: U.S. troops entered Mexico City and effectively ended the Mexican War.
- December 6: Abraham Lincoln took his seat in the U.S. House of Representatives. After serving a single two-year term, he returned to Illinois.
- January 24: James Marshall, a mechanic at John Sutter's sawmill in northern California, recognized some unusual nuggets. His discovery would set off the California Gold Rush.
- February 23: Former president John Quincy Adams, who served as a U.S. Congressman from Massachusetts after leaving the presidency, died after collapsing in the U.S. Capitol building.
- July 12-19: A conference at Seneca Falls, New York, organized by Lucretia Mott and Elizbeth Cady Stanton, took up the issue of Women's Rights and planted the seeds of the suffrage movement in the U.S.
- November 7: Zachary Taylor, Whig candidate and a hero of the Mexican War, was elected President of the United States.
- December 5: President James Knox Polk, in his annual address to Congress, confirmed the discovery of gold in California.
- March 5: Zachary Taylor was inaugurated as the 12th president of the U.S. He was the third, and last, candidate of the Whig Party to hold the office.