Benton was born in New Market, Maryland. His father died when he was four months old and in 1836, he moved with his mother to Richmond, Indiana. Beginning at age 15, Benton spent 2–3 years in Cincinnati as a chairmaker. When Benton was 18 years old, he enlisted as a private in the Mexican War, and fought with gallantry in the mounted infantry at Contreras, Churubusco, Chapultepec and Mexico City. Returning to Richmond at the war's end, he entered college to study law. He was admitted to the bar in Indiana in 1851 and began practice with Charles Clark. In 1852, he was elected district attorney of Wayne County on the Whig ticket and served until 1854. In 1854, he formed a partnership with J. B. Julian which continued until 1856, when he was elected judge of the Common pleas court, serving one term, until defeated for renomination in 1858. In 1855, Benton was married to Sarah A. Wiggins, daughter of Daniel A. Wiggins of Richmond. He and Sarah had three children, Walter, Jessie, and Mary. His wife died of consumption in 1861 at the age of 27.
Civil war
Benton raised the first company from Wayne County following Lincolns call for 75,000 volunteers. He was its captain when the company became part of the 8th Indiana Volunteer Infantry Regiment, but was elected the regiment's colonel. He led the 8th Indiana in some the earliest fighting of the war during McClellan'sWestern Virginia campaign of 1861, including the Battle of Rich Mountain. The regiment was then ordered to Missouri. Benton is said to have commanded a brigade at the Battle of Pea Ridge, although the Official Records lists another officer as the official commander, a member of the 33rd Illinois Volunteer Infantry Regiment described as "lacking in the fundamental requisites of leadership". While in Missouri, he was remarried to a war widow, Emma Adolphin Lenhart, after a ten-day courtship. At the Battle of Cotton Plant in July 1862, he led a brigade. His troops came on the field after most of the fighting was over and helped rout the beaten Confederate force. In April 1862 Benton was promoted to brigadier general of volunteers. He served with distinction at the Battles of Port Gibson, Jackson, Champion Hill, Big Black River, and at the Siege of Vicksburg. He then served in various district commands with the XIII Corps in Texas and Louisiana throughout 1864, until he commanded a division in the campaign against Mobile, Alabama in early 1865. Benton was mustered out of the volunteer service on July 24, 1865. On January 13, 1866, President Andrew Johnson nominated Benton for the award of the brevet grade of major general of volunteers, to rank from March 26, 1865, and the U.S. Senate confirmed the award on March 12, 1866.
