Sarah Edmonds was born in New Brunswick, Canada, in December, 1841. Her father wanted a son and treated her very badly as a child. When she was a teenager she ran away from home and found work as an itinerant Bible seller. She eventually arrived in the United States and settled in Flint, Michigan.
On 25th May, 1861, Edmonds, using the alias of Franklin Thompson, enlisted as a private in the Second Michigan Infantry in Detroit. While in the Union Army she participated in the Peninsula campaign and the battles of Bull Run (August, 1862), Antietam (September, 1862) and Fredericksburg (December, 1862).
In December, 1862, Colonel Orlando Poe, an officer serving under General George McClellan, sent Edmonds on a spying mission behind Confederate lines. Edmonds disguised herself as a black man and worked building fortifications for the Confederate Army at Yorktown. After three days she returned to the Union Army and provided valuable information on the enemy.
Poe was so pleased with the success of the mission that he sent her out again in February, 1863. This time she disguised herself as an Irish peddler with the name Bridget O'Shea. She sold goods to the Confederate Army and as well as gaining valuable information also stole a beautiful horse that she called Rebel.
Soon afterwards her regiment was sent to join the troops under General Ulysses Grant, who was about to start the important Vicksburg Campaign. Edmonds worked in the military hospital looking after wounded and sick soldiers. While nursing in the hospital Edmonds developed malaria. Aware that her treatment would result in the discovery of her gender, she decided to desert from the army.
Edmonds went to Washington and found work as a nurse in a hospital for wounded soldiers run by the United States Christian Commission. After the war Edmonds published an account of her experiences, Nurse and Spy in the Union Army (1865).
In 1886 she received a government pension based upon her military service. A letter from the secretary of war acknowledged her as "a female soldier who served as a private rendering faithful service in the ranks."