Cornelia Hancock was born in Hancock's Bridge, New Jersey, on February 8, 1840. Her parents, Thomas Yorke and Rachel Nicholson Hancock, were Quakers and her father was a fisherman by trade. Hancock had a brother William and a sister Ellen (m. Dr. Henry Child of Philadelphia). William served in two regiments during the war: the 24th Regiment, New Jersey Infantry, and the 37th Regiment, New Jersey Volunteers. During his time in the 37th, William and Cornelia were located in the same area and were able to meet on several occasions.
At the outbreak of the Civil War, Cornelia was motivated to assist the Union effort after seeing her brother and other male relatives and friends leave for the war. Advised by her brother-in-law that assistance was needed near Gettysburg in July 1863, Hancock volunteered as a nurse but was denied because she was too young. Undaunted, she made her own way to the battlefield and arrived just after the conclusion of the fighting, quickly winning admiration from both soldiers and staff for her work with the wounded. During the winter of 1863-1864, Hancock moved to Washington, D.C., and spent the winter working with displaced African Americans at the Contraband Hospital. Her work impressed Secretary of War Edwin Stanton, who gave Hancock a pass allowing her to travel wherever necessary within the lines of the Army of the Potomac. In February 1864, she moved to the field hospital of the 3rd Division of the 2nd Corps, near Brandy Station, Virginia. She assisted the wounded after the Battle of the Wilderness and, in May 1865, followed the 1st Division, 2nd Corps, on its march through Virginia. While with the 2nd Corps, Hancock arrived at City Point Hospital, Virginia, in June 1864, where she worked throughout the remainder of the war.
After the war, Hancock continued her charitable work, moving to South Carolina in 1866 to care for newly freed slaves. During her time there, she founded the Laing School for Negroes (Pleasantville, South Carolina) with the assistance of her friend Laura Towne, using funds from the Freedmen's Bureau and donations from the Philadelphia Yearly Meeting of the Society of Friends. Around 1876, Hancock traveled to England with some friends, and by 1878 she had returned to Philadelphia, where she and Dr. Henry Child, her brother-inlaw, founded the Society for Organizing Charity (later the Family Society of Philadelphia). Hancock's other charitable work includes the founding, in 1884, of the Children's Aid Society and Bureau of Information (later the Children's Aid Society of Philadelphia), and her work with the poor residents of Wrightsville, an isolated working-class community in south Philadelphia. Hancock remained active in her charitable work until late in her life, and served as the secretary for the Association of Army Nurses of the Civil War in the early 20th century, by which time she had returned to New Jersey and was living in Atlantic City.
A collection of Cornelia Hancock's Civil War-era letters was first published in 1937 as South After Gettysburg: Letters of Cornelia Hancock, and the same collection remains in print today as Letters of a Civil War Nurse. The published letters from 1863-1865 are almost universally represented in the collection, with the published post-war items drawn from other sources. The Cornelia Hancock papers at the Clements Library hold many items not in the published editions.