The first few months were spent on the Mississippi river guarding Cairo. Their three-month enlistments were up in July. Most of the men reenlisted for three years. In September the regiment moved to Paducah, Kentucky. There they did little more than drill practice. On February 15, 1862 they joined the fighting at the Battle of Fort Donelson. Their losses included 36 killed and 165 wounded, out of a total strength of 600. After the Confederates surrendered, they were transported by riverboats up the Tennessee River. They left the boats at Pittsburg Landing, Tennessee and camped nearby. On April 6, 1862 they woke up to the sound of cannons. While they were waiting to join the battle, new orders were given. They were ordered to exchange their gray coats for Union blue. This way they would not be mistaken for Confederate troops. They then joined the battle of Shiloh. They fought most of the day. At Shiloh the 9th Illinois had the highest casualties of any Illinois unit. They lost 366 of their comrades They also fought at the Second Battle of Corinth. The regiment was changed to the 9th Illinois Volunteer Mounted Infantry Regiment on March 15, 1863. On July 9, 1865 the 9th Illinois Infantry was mustered out (released from military service) at Louisville, Kentucky.