In 1859, he attended Butler University in Indianapolis, leaving after one year before to enlist in the Union Army following the outbreak of the Civil War.
After enlisting in Company B of the 7th Indiana Infantry Regiment, Hadley was sent to Camp Morton in Indianapolis for a short period of training and was appointed corporal.
In 1862, he was promoted to sergeant before being wounded at the Second Battle of Bull Run and was briefly taken as a prisoner of war by Confederate soldiers before being let free.
Returning to Hendricks County, Hadley married Mary Jane Hill, a woman he had been sending love letters to throughout the war.
His judicious and professional handling of a case that was monitored so closely by the press brought Hadley to the attention of the public, leading to his election to the Indiana Supreme Court.
Their son, Walter Gresham Hadley, served as an artillery captain in the American Expeditionary Force during the First World War.