Events from the year 1861 in the United States. This year marked the beginning of the American Civil War.
- President: James Buchanan (D-Pennsylvania) (until March 4), Abraham Lincoln (R-Illinois) (starting March 4)
- Vice President: John C. Breckinridge (D-Kentucky) (until March 4), Hannibal Hamlin (R-Maine) (starting March 4)
- Chief Justice: Roger B. Taney (Maryland)
- Speaker of the House of Representatives: William Pennington (R-New Jersey) (until March 4), Galusha A. Grow (R-Pennsylvania) (starting July 4)
- Congress: 36th (until March 4), 37th (starting March 4)
- January 3 – American Civil War: Delaware votes not to secede from the Union.
- January 9 – Mississippi becomes the second state to secede from the Union, preceding the American Civil War.
- January 10 – American Civil War: Florida secedes from the Union.
- January 11 – American Civil War: Alabama secedes from the Union.
- January 12 – Major Robert Anderson sends dispatches to Washington.[clarification needed]
- January 12 – American Civil War: Florida state troops demand surrender of Fort Pickens.
- January 18 – American Civil War: Georgia secedes from the Union.
- January 21 – American Civil War: Jefferson Davis resigns from the United States Senate. Ordinance of Secession is ratified.
- January 26 – American Civil War: Louisiana secedes from the Union.
- January 29 – Kansas is admitted as the 34th U.S. state (see History of Kansas).
- February 1 – American Civil War: Texas secedes from the Union.
- February 4 – American Civil War: Delegates from six seceded states meet at the Montgomery Convention in Montgomery, Alabama.
- February 8 – American Civil War: The Confederate States of America adopts the Provisional Constitution of the Confederate States.
- February 9 – American Civil War: Jefferson Davis is elected the Provisional President of the Confederate States of America by the Weed Convention at Montgomery, Alabama.
- February 11 – American Civil War: The U.S. House unanimously passes a resolution guaranteeing non-interference with slavery in any state.
- February 18 – American Civil War: In Montgomery, Alabama, Jefferson Davis is inaugurated as the provisional president of the Confederate States of America.
- February 23 – President-elect Abraham Lincoln arrives secretly in Washington, D.C. after an assassination attempt in Baltimore.
- February 28 – Colorado Territory is organized.
- March 2 – Nevada Territory and Dakota Territory are organized.
- March 4
- March 11 – American Civil War: The Constitution of the Confederate States is adopted.
- April 12 – Battle of Fort Sumter: The American Civil War begins at Fort Sumter, South Carolina.
- April 14 – Battle of Fort Sumter: Fort Sumter surrenders to Confederate forces.
- April 17 – The state of Virginia secedes from the Union.
- April 20 – American Civil War: Robert E. Lee resigns his commission in the United States Army in order to command the forces of the state of Virginia.
- April 25 – American Civil War: The Union Army arrives in Washington, D.C.
- April 27 – American Civil War:
- May 6 – American Civil War: Arkansas secedes from the Union.
- May 7 – American Civil War: Tennessee secedes from the Union.
- May 8 – American Civil War: Richmond, Virginia is named the capital of the Confederate States of America.
- May 10 – American Civil War – Camp Jackson Affair: Union military forces clash with civilians on the streets of St. Louis, Missouri, resulting in the deaths of at least 28 people and injuries to another 100.
- May 13 – American Civil War: Queen Victoria issues a "proclamation of neutrality" which recognizes the breakaway states as having belligerent rights.
- May 20
- July 13 – American Civil War: The Battle of Corrick's Ford takes place in western Virginia.
- July 21 – American Civil War – First Battle of Bull Run aka First Manassas: At Manassas Junction, Virginia, the first major battle of the war ends in a Confederate victory.
- July 22 – American Civil War: After Union forces led by Nathaniel Lyon capture the Missouri state capital of Jefferson City, the Missouri Constitutional Convention reconvenes and removes pro-secessionist Governor Claiborne Fox Jackson from office, replacing him with a pro-Union governor.
- July 25 – American Civil War: The Crittenden–Johnson Resolution is passed by the U.S. Congress, stating that the war is being fought to preserve the Union and not to end slavery.
- July 26 – American Civil War: George B. McClellan assumes command of the Army of the Potomac following a disastrous Union defeat at the First Battle of Bull Run.
- August 5
- August 10 – American Civil War: The first major battle west of the Mississippi River, the Battle of Wilson's Creek, is fought, with a Confederate victory.
- September 3 – American Civil War: Confederate General Leonidas Polk invades neutral Kentucky, prompting the state legislature to ask for Union assistance.
- September 6 – American Civil War: Forces under Union General Ulysses S. Grant bloodlessly capture Paducah, Kentucky, which gives the Union control the mouth of the Tennessee River.
- October 9 – American Civil War – Battle of Santa Rosa Island: Confederate forces are defeated in their effort to take the island.
- October 21 – American Civil War – Battle of Ball's Bluff: Union forces under Colonel Edward Baker are defeated by Confederate troops in the second major battle of the war. Baker, a close friend of Abraham Lincoln, is killed in the fighting.
- October 28 – A small pro-secessionist section of the Missouri legislature takes up a bill for Missouri's secession from the Union.
- October 30 – The bill is passed for Missouri's secession from the Union.
- October 31
- Missouri's secession from the Union bill is signed by Governor Claiborne Fox Jackson, but by this date Governor Jackson only controls parts of South-Western Missouri. Union forces led by general John C. Fremont have consolidated control over the vast majority of Missouri.
- American Civil War: Citing failing health, Union General Winfield Scott resigns as Commander of the United States Army.
- November 1 – American Civil War: U.S. President Abraham Lincoln appoints George B. McClellan as commander of the Union Army, replacing the aged General Winfield Scott.
- November 2 – American Civil War: Western Department Union General John C. Frémont is relieved of command and replaced by David Hunter.
- November 6 – American Civil War: Jefferson Davis is elected president of the Confederate States of America.
- November 7 – American Civil War – Battle of Belmont: In Mississippi County, Union forces led by General Ulysses S. Grant overrun a Confederate camp but are forced to retreat when Confederate reinforcements arrive.
- November 8 – American Civil War – Trent Affair: The USS San Jacinto stops the United Kingdom mail ship Trent and arrests two Confederate envoys, James Mason and John Slidell, sparking a diplomatic crisis between the U.K. and U.S.
- November 21 – American Civil War: Confederate President Jefferson Davis appoints Judah Benjamin Secretary of War.
- November 28 – American Civil War: Acting on the ordinance passed by the Jackson government, the Confederate Congress admits Missouri as the 12th Confederate state.
- December 10 – American Civil War: Kentucky is accepted into the Confederate States of America.
- Alonzo E. Deitz founds the A. E. Deitz lock company in Brooklyn, New York.
- January 7 – Louise Imogen Guiney, poet (died 1920)
- January 12 – James Mark Baldwin, philosopher and psychologist (died 1934)
- January 26 – Frank O. Lowden, 25th Governor of Illinois from 1917 and U.S. Representative from Illinois from 1906 to 1911 (died 1943)
- January 29 – William M. Butler, U.S. Senator from Massachusetts from 1892 to 1895 (died 1937)
- February 26 – Godfrey Lowell Cabot, industrialist and philanthropist (died 1962)
- March 1 – Henry Harland, novelist and editor (died 1905)
- March 15 – Joseph M. Devine, 6th Governor of North Dakota from 1898 to 1899 (died 1938)
- March 20 – Wilds P. Richardson, U.S. Army officer (died 1929)
- April 17 – Willard Saulsbury, Jr., U.S. Senator from Delaware from 1913 to 1919 (died 1927)
- April 19 – John Grier Hibben, minister, philosopher and educator (died 1933)
- April 20 – James D. Phelan, U.S. Senator from California from 1915 to 1921 (died 1930)
- April 23 – John Peltz, baseball player (died 1906)
- April 27 – William Lorimer, U.S. Senator from Illinois from 1909 to 1912 (died 1934)
- May 16 – Herman Webster Mudgett, alias H. H. Holmes, serial killer (died 1896)
- May 20 – Henry Gantt, project engineer (died 1919)
- May 25 – Julia Boynton Green, poet (died 1947)
- June 2 – Helen Herron Taft, First Lady of the U.S. as wife of 27th President William Howard Taft (died 1943)
- June 6 – Joseph M. Terrell, U.S. Senator from Georgia from 1910 to 1911 (died 1912)
- June 29 – William James Mayo, physician, medic, co-founder of Mayo Clinic (died 1939)
- July 7 – Nettie Stevens, geneticist (died 1912)
- July 9 – James M. Beck, politician (died 1936)
- July 11 – George W. Norris, U.S. Senator from Nebraska from 1913 till 1943 (died 1944)
- July 22
- July 26 – James K. Vardaman, politician (died 1930)
- August 3 – Samuel M. Shortridge, U.S. Senator from California from 1921 till 1933 (died 1952)
- August 4 – Jesse W. Reno, inventor, builder of the first working escalator (died 1947)
- August 6 – Edith Roosevelt, née Carow, First Lady of the U.S. (died 1948)
- August 9
- August 20 – Anna Shelton, businesswoman (died 1939)
- September 20 – Herbert Putnam, Librarian of Congress (died 1955)
- September 21 – L. Heisler Ball, U.S. Senator from Delaware from 1919 to 1925 (died 1932)
- September 30 – William Wrigley, Jr., chewing gum industrialist (died 1932)
- October 4 – Frederic Remington, painter, illustrator, sculptor and writer (died 1909)
- October 19 – William J. Burns, detective and director of Bureau of Investigation (died 1932)
- November 2 – Charles W. Waterman, U.S. Senator from Colorado from 1927 to 1932 (died 1932)
- November 6
- November 10 – Bessie Alexander Ficklen, doggerel poet and hand puppet specialist (died 1945)
- November 14 – Frederick Jackson Turner, historian (died 1932)
- November 26 – Albert B. Fall, U.S. Senator from New Mexico from 1912 to 1921 and Secretary of the Interior from 1921 to 1923 under President Warren G. Harding (died 1944)
- December 8 – William C. Durant, businessman (died 1947)
- December 15 – Charles Duryea, engineer and manufacturer of motor vehicles (died 1938)
- April 4 – John McLean, U.S. Postmaster General from 1823 to 1829, Associate Justice of the Supreme Court from 1829 to 1861 (born 1785)
- April 8 – Elisha Otis, industrialist, founder of the Otis Elevator Company (born 1811)
- April 15 – Isaiah Stillman, U.S. Army Major in the Black Hawk War (born 1793)
- May 21 – Benjamin Paul Akers, sculptor (born 1825)
- May 24 – Elmer E. Ellsworth, first Union officer to die in the Civil War (born 1837)
- June 3 – Stephen A. Douglas, Senator from Illinois from 1847 till 1861 and Presidential candidate (born 1813)
- June 5 – John Garland, Bvt. Brigadier General in the Union Army (born 1793)
- June 13 – Richard Lawrence, failed assassin of Andrew Jackson (born c. 1800–1801)
- July 7 – John Willis Ellis, 35th Governor of North Carolina from 1859 to 1861 (born 1820)
- July 13 – Robert S. Garnett, Confederate brigadier general (born 1819)
- July 22 – Barnard Elliott Bee, Jr., Confederate general (born 1824)
- August 10 – Nathaniel Lyon, Union Army brigadier general, first general to be killed in the Civil War (born 1818)
- August 12 – Eliphalet Remington, gunmaker (born 1793)
- August 17 – Alcée Louis la Branche, politician (born 1806)
- October 5 – Kinsley S. Bingham, U.S. Senator from Michigan from 1859 to 1861 (born 1808)
- October 20 – William Woodbridge, Governor of Michigan from 1840 to 1841 and U.S. Senator from Michigan from 1841 to 1847 (born 1780)
- October 21 – Edward Dickinson Baker, U.S. Senator from Oregon from 1860 to 1861 (born 1811)
- October 26 – Edward "Ned" Kendall, bandleader and instrumentalist (keyed bugle) (born 1808)
- November 28 – Richard M. Young, U.S. Senator from Illinois from 1837 to 1843 (born 1798)
- "Historical Events in January 1861". OnThisDay.com. Retrieved 6 December 2018.
- American Annual Cyclopaedia ... 1861, NY: D. Appleton & Co. – via HathiTrust
- Media related to 1861 in the United States at Wikimedia Commons