Edmonds' interest in adventure was sparked in childhood by Maturin Murray Ballou's book "Fanny Campbell, the Female Pirate Captain", telling the story of Fanny Campbell and her adventures on a pirate ship during the American Revolution while dressed as a man. Campbell continued dressing as a man after the war in order to pursue other adventures. Edmonds used Campbell as an inspiration to "escape the limitations of her sex." She enlisted in Company F of the 2nd Michigan Infantry on May 25, 1861, also known as the Flint Union Greys. She disguised herself as a man named Franklin Flint Thompson, the middle name possibly after the city of Flint, Michigan where she volunteered. She felt that it was her duty to serve the United States, as it was her new country. She at first served as a field nurse, participating in several campaigns under General McClellan, including the First and Second Battle of Bull Run, Antietam, the Peninsula Campaign, Vicksburg, Fredericksburg, and others.