Sickles learned the printer's trade, studied in the University of the City of New York, was admitted to the bar in 1846, and was a member of the New York Assembly in 1847. In 1853, he became corporation counsel of New York City, but resigned soon afterward to become secretary of the U.S. legation in London, under James Buchanan, by appointment of President Franklin Pierce. He returned to America in 1855, was a member of the senate of New York state from 1856-57, and from 1857-61 was a Democratic representative in the U.S. Congress.
In 1859, Sickles became the first man acquitted of a murder charge on the grounds of temporary insanity. While a congressman, he shot down Philip Barton Key. Key was the son of Francis Scott Key and U.S. Attorney for the District of Columbia. Sickles had discovered that Key was having a blatantly public affair with his wife, Teresa B. Sickles. Defense attorney Edwin M. Stanton, the future U.S. Secretary of War, gained the innovative verdict. Sickles withdrew briefly from public life due to the notoriety of the trial. He publicly forgave his wife, outraging the public, which had applauded his role in the shooting, and apparently ending his political career.
At the start of the Civil War, Sickles desired to repair his public image and was active in raising U.S. volunteers in New York. He was appointed Colonel of one of the regiments he organized. He was promoted to Brigadier General of volunteers in September 1861, becoming one of the most famous "political generals" in the Union army. Despite his complete lack of previous military experience, he did a competent job of commander in the Army of the Potomac during the Peninsula Campaign and the Battle of Antietam, and then succeeded to a divisional command. Frequently absent from his command seeking advancement in Washington, D.C., he nonetheless commanded his brigade at the Battle of Seven Pines and during the Seven Days Battles.
Sickles was a close ally of Major Gen. Joseph Hooker. Both men had notorious reputations as political climbers and as hard-drinking ladies' men. Accounts at the time compared their army headquarters with a rowdy bar and bordello.
In charge of the Division, Sickles fought at the Battle of Fredericksburg and received the regular army brevet of Brigadier General in 1867. His prewar reputation as a womanizer and heavy drinker returned to him during his career as a brigade and division commander. His heyday came when Major Gen. Joseph Hooker took command of the army. Many officers complained that Hooker, Sickles, and Major Gen. Daniel Butterfield had converted the army headquarters into a combination of bar and brothel. Sickles' own headquarters were considered to be even worse. After fighting at the Battle of Chancellorsville, Sickles retained charge of the III Corps after Hooker's removal.
The Battle of Gettysburg marked the most famous incident, and the effective end, of his military career. On July 2, Major Gen. George G. Meade ordered Sickles' corps to take up defensive positions on the southern end of Cemetery Ridge, anchored in the north to the II Corps and to the south, Little Round Top. He was unhappy to see a slightly higher terrain feature to his front, the Peach Orchard. Sickles violated his orders and marched his corps almost a mile in front of Cemetery Ridge. Meade confronted Sickles about his insubordination, but it was too late. The Confederate assault by Major Gen. James Longstreet's corps smashed the III Corps and rendered it useless for further combat.
Sickles fell victim to a cannonball that mangled his right leg. His leg was amputated that afternoon and he insisted on being transported back to Washington, D.C., which he reached on July 4, bringing some of the first news of the great Union victory, and starting a public relations campaign to ensure his version of the battle prevailed. He preserved the leg's bones and donated them to the Army Medical Museum, along with a visiting card marked, "With the complements of Major General D.E.S." For several years thereafter, he reportedly visited the limb on the anniversary of the amputation.
Sickles ran a vicious campaign against Meade's character after the Civil War. He felt that Meade had wronged him at Gettysburg and that credit for winning the battle belonged to him. In testimony before a congressional committee, Sickles maintained that Meade had secretly planned to retreat from Gettysburg on the first day. Sickles said that his movement away from Cemetery Ridge may have violated orders, but it was the correct move because it disrupted the Confederate attack, redirecting its thrust, effectively shielding their real objective, Cemetery Hill.
As a result, he was denied further field command and was assigned a series of special missions by the War Department. He was awarded the Medal of Honor for his actions, although it took 34 years to get it. Made Colonel, 42nd Infantry, in the 1866 regular army reorganization, he was mustered out of the volunteer service as a major general on January 1, 1868.
After the Civil War, Sickles was sent on a confidential mission to Colombia (the "special mission to the South American Republics") to secure its compliance with a treaty agreement of 1846, permitting the United States to convey troops across the Isthmus of Panama. From 1865-67, he commanded the Department of South Carolina, the Department of the Carolinas, the Department of the South, and the Second Military District. In 1866, he was appointed Colonel of the 42nd U.S. Infantry (Veteran Reserve Corps), and in 1869, he was retired with the rank of major general.
In 1869, Sickles retired with the advanced rank of major general in the regular army. He was appointed the U.S. minister to Spain, from 1869-73, by President Ulysses S. Grant. He was rumored to have had an affair with Spain's deposed Queen Isabella II. In 1871, he married again, following the death of Teresa in 1867, to Senorita Carmina Creagh, the daughter of Chevalier de Creagh of Madrid, a Spanish Councillor of State, and had 2 children.
Sickles was president of the New York State Board of Civil Service Commissioners in 1888-89, sheriff of New York in 1890, and was again a representative in Congress from 1893-95. In the 1890's, he served a term in Congress.
For 26 years, until forced out in a financial scandal, he was the chairman of the New York State Monuments Commission. He helped to lead the preservation efforts at the Gettysburg Battlefield, sponsoring legislation to form the Gettysburg National Military Park.
Sickles proved to be one of the most controversial of Union corps commanders. He is buried in Arlington National Cemetery.
Colonel - June 20, 1861
Brigadier General USV- September 3, 1861
Major General USV- November 29, 1862
Colonel, 70th New York
2nd ("Excelsior") Brigade, Hooker's Division, Army of the Potomac (October 3, 186 1-March 13, 1862)
2nd ("Excelsior") Brigade, 2nd Division, III Corps, Army of the Potomac (May 24 - July 16, 1862)
2nd Division (September 5, 1862 - January 12, 1863)
III Corps (February 5 - May 29 and June 3 - July 2, 1863)