ICONS: Mexican-American War, Sewing Machine, Gold Rush

- Captain Charles Wilkes circumnavigates Antarctica, claiming it for the United States on January 19, 1840.
- William Henry Harrison wins the 1840 presidential election, defeating incumbent Martin Van Buren.
- The August 9, 1842, Webster-Ashburton Treaty formally designates the border separating the United States and Canada.
- On May 24, 1844, Samuel B. Morse sends the message, "What hath God wrought" over the first telegraph line from Washington, DC, to Baltimore, MD.
- Ireland's "Potato Famine" results in an influx of Irish immigrants to the United States beginning in 1845.
- Alexander Cartwright and his New York Knickerbockers codify the "Rules of Baseball" in 1845.
- Frederick Douglass publishes Narrative of the Life of Frederick Douglass, an American Slave in 1845.
- Elias Howe patents the sewing machine on September 10, 1846.
- During the February 22-23, 1847, Battle of Buena Vista, General Taylor's U.S. Army of 4800 men defeats Mexico's 15,000-man force lead by General Santa Anna.
- The American Medical Association is founded in Philadelphia on May 7, 1847.
- On January 24, 1848, James Marshall discovers gold near Sutter's Fort, California. The discovery of gold is the beginning of the California Gold Rush of 1849.
- The 1849 Treaty of Guadalupe Hidalgo ends the Mexican-American War. In exchange for $15 million and settling $3.25 million in American claims, Mexico cedes approximately 500,000 square miles of territory in the western and southwestern United States.
- Elizabeth Blackwell receives the first medical degree awarded to a woman on January 23, 1849, from the Medical Institute of Geneva, NY.
- Henry David Thoreau publishes "Resistance to Civil Government," later known as "Civil Disobedience," in 1849.