In the Spring of 1862 Garfield and his new command of the 20th Brigade of the Army of the Ohio, were sent to Shiloh. Unfortunately for Garfield and his men they would receive no glory here for they arrived on the battle seen only in time to see the last of the Confederates running from the field.

The battles won on the field bolstered his reputation at home enough to have his friends in the 19th District nominate him for Congress. Garfield, although still in the army, had been sent back to Ohio to recuperate from illness. Garfield, in keeping with his tradition of not appearing to seek public office, did not campaign but his friends had certainly done their part and he was elected.

Although Garfield had been elected to Congress the term would not begin until December of 1863. With the war still raging Garfield was sent to Washington on military duty and was looking for a new command. Garfield by that time had reached the rank of brigadier general but Washington at that time was full of brigadier generals and he was hard pressed to get a command.

During that time in Washington Garfield struck up a friendship with the former Governor of Ohio and current Secretary of the Treasury Salmon P. Chase. It was around the Chase dinner table that Garfield got his first lessons in the financial world, which later served him well in Congress.

Finally in January of 1863 Garfield was given an assignment in the Army of the Cumberland under the Command of the somewhat eccentric General William S. Rosencrans. Rosencrans with his seamlessly endless supply of energy would often come into Garfield's tent and talk until the early hours of the morning. The two men struck up a close friendship and Rosencrans would then name Garfield his chief of staff. Unfortunately for Garfield, Rosencrans developed what many Union generals had developed a fear of attacking the enemy. Rosencrans was constantly seeking more supplies and men from Washington, but would not move the army against the enemy.

It wasn't until June of 1863, encouraged endlessly by Garfield and the War Department, that Rosencrans finally moved his army against General Bragg. Garfield helped to put together a battle plan that if successfully carried out could quite possibly have eliminated Bragg's army. Unfortunately execution of the plan and the weather played a roll in minimizing the effectiveness of the campaign. For, although the Confederates were forced to give ground, Bragg's army survived. Once again Rosencrans failed to follow up on his victory and at that point he began to lose the admiration of Garfield.

Chickamauga Creek would spell the end for Rosencrans. His army was rolled up by reinforced Confederates and Rosencrans was unable to act. He left the field in flight with most of his army. During the battle Garfield would not leave the field until he knew the condition of General Thomas and his forces to the far left of the Union line. While Rosencrans was in full retreat, Garfield made a death-defying ride to get to Thomas. None of the aides who began the ride with Garfield survived. Even the horse, which Garfield rode, dropped dead when he reached Thomas. Thomas held his position and Rosencrans was removed from his command a broken man.

By December of 1863 Garfield's term in Congress was set to begin. He had been promoted to major general and he felt with the war going on his place was still with the army. Lincoln intervened, however, and told Garfield that he had an over abundance of commanding generals but needed congressman with Garfield's administrative capabilities and knowledge of the workings of the army. Garfield took Lincoln's advice and resigned his commission. Before Garfield took office, however, he suffered the tragic loss of his first child, three year old Eliza, on December 1, 1863.