Ballou was born the son of Hiram and Emeline (Bowen) Ballou, a distinguished Huguenot family in Smithfield, Rhode Island. He lost both of his parents at a young age and was forced to fend for himself. In spite of this, he attended boarding school at Phillips Academy in Andover, Massachusetts. Following his graduation therefrom, he attended Brown University, where he was a member of Delta Phi, and went on to study law at the National Law School, in Ballston, New York. He was admitted to the Rhode Island bar and began to practice in 1853.
Ballou was active in public service. Shortly after being admitted to the bar, he was elected to the Rhode Island House of Representatives, where he served as a clerk, and later as the speaker. He was a staunch Republican and supporter of Abraham Lincoln. When war broke out, Ballou immediately left what appeared to be a promising political career and volunteered for military service with the 2nd Rhode Island Infantry. In addition to his combat duties, he served as the Rhode Island militia's judge advocate.
Ballou and 93 of his men were mortally wounded at Bull Run. In an attempt to better direct his men, Ballou took a horse mounted position in front of his regiment, when a 6-pounder solid shot from Confederate artillery tore off his right leg and simultaneously killed his horse. The badly injured Major was then carried off the field and the remainder of his leg was amputated. Ballou died from his wound a week after that Union defeat and was buried in the yard of nearby Sudley Church. After the battle the territory was occupied by Confederate forces. According to witness testimony, it was at this time that Ballou's corpse was exhumed, decapitated, and desecrated by Confederate soldiers possibly belonging to the 21st Georgia regiment. Ballou's body was never recovered.
In place of his body, charred ash and bone believed to be his remains were reburied in Swan Point Cemetery, Providence, Rhode Island. His wife, Sarah, never remarried. She later moved to New Jersey to live out her life with a son, William. She died in 1917 and is buried next to her husband. Ballou married Sarah Hart Shumway on October 15, 1855. They had two sons, Edgar and William. In his letter to his wife, Ballou attempted to crystallize the emotions he was feeling: worry, fear, guilt, sadness and, most importantly, the pull between his love for her and his sense of duty.
The letter was featured prominently in the Ken Burns documentary The Civil War, where it was paired with Jay Ungar's musical piece "Ashokan Farewell" and read by Paul Roebling. However, the documentary featured a shortened version of the letter, which did not contain many of Ballou's personal references to his family and his upbringing. It has been difficult to identify which of the several extant versions is closest to the one he actually wrote, as the original seems not to have survived. The following is an extended version: