In May 1861, Brownell and his regiment, known as the 11th Fire Zouaves—because they were New York City firefighters—arrived in Washington, and were mustered into federal service. On May 24, the Zouaves crossed the Potomac River into Alexandria, Va. When the regimental commander, Col. Elmer Ellsworth, was shot dead while attempting to take down a Confederate flag, Brownell shot the innkeeper who killed his colonel. In 1877 he received the Medal of Honor for his actions.