Hancock was a brigadier general at the beginning of the American Civil War in 1861 fighting for the Union, and commanded the 2nd Corps at the battle of Gettysburg on July 1-3, 1863. At Gettysburg he was given command of the Union forces after General John Reynolds was killed early on the 1st, until George Meade, the commander of the Army of the Potomac, arrived late that night--a great honor, as Hancock was not the most senior Union officer at Gettysburg at the time. Wounds incurred by Hancock at Gettysburg caused him to spend less time as a field commander late in the Civil War, and his growing dissatisfaction with Grant's casualty-intensive tactics lessened his enthusiasm for command during those months when his health returned. After the war, Hancock served as the major general in command of the Department of the East, headquartered at Governors Island, New York. Immediately following the Civil War, Hancock drew much criticism for his inclination to be lenient to the defeated Confederates.
Hancock was considered but passed over for the Democratic nomination for U.S. President during the 1870s. He was eventually chosen as the Democratic opponent to James Garfield in the U.S. election of 1880, but was narrowly defeated in his attempt.
He died on February 9, 1886, at Governor's Island, still in command of the Department of the East. He is buried in Mongomery Cemetery in Norristown, Pennsylvania.