The father was remembered by a participant in that battle, Adjutant General Richard Coulter Drum of the Regular Army, as "by all odds the most brilliant man under fire I have ever seen."

He initially lived with his uncle, George Gilson, then mayor of Peru, an Illinois River town in LaSalle County.

During that period, he was joined by his close friend and fellow Norwich University graduate, Grenville M. Dodge, who would later win fame as a Civil War general and the chief engineer of the Union Pacific Railroad.

As the Civil War began, Ransom was in the employ of the Illinois Central Railroad, living in Fayette County.

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.