In response to President Abraham Lincoln's call for troops in 1861, Ransom raised a body of soldiers that became Company E of the 11th Illinois Infantry, and was elected its captain on April 6, 1861, then major on June 4. He was commissioned lieutenant colonel of the regiment July 30, and colonel on February 15, 1862. He was commissioned brigadier general on November 9, 1862, and in January 1863, took command of a brigade in Brigadier General John McArthur's Sixth Division of McPherson's XVII Corps.