At the outbreak of the American Civil War, Bryant volunteered for service with a Wisconsin militia company and was enrolled in the 3rd Wisconsin Infantry Regiment. He declined an officer's commission, but at the organization of the regiment, he was appointed sergeant major. Within a year, he received a commission anyway and was appointed adjutant to the regiment's colonel, Thomas H. Ruger. With the 3rd Wisconsin Infantry, Bryant participated in many of the significant battles of the eastern theater of the war, including Antietam, Chancellorsville, and Gettysburg. In August of 1863, he was with the brigade sent to quell the New York City draft riots.