Augustus Henry Seward (he went by Gus in close circles) was born October 10, 1826 to William Henry Seward and Frances Adeline Seward (née Miller) in Auburn, New York. He was the eldest of five children. His siblings were Frederick William Seward, William Henry Seward Jr., Cornelia Seward (who died in infancy), and Frances Adeline Seward.

Gus graduated from West Point on June 30, 1847, ranking 34th out of 38 students. After graduation, Gus was appointed to Second Lieutenant of the 8th Infantry Regiment on July 1, 1847. He served in this position throughout the Mexican American War. After the war, Gus was transferred to the 5th Infantry Regiment on September 13, 1847. He was promoted to First Lieutenant on April 30, 1853, and later Captain on January 19, 1859. Gus was appointed to Paymaster on March 27, 1861, and served in this position throughout the Civil War. After the war, he continued to serve as a Paymaster in multiple locations such as Boston and New York.

Gus was injured by Lewis Powell who attempted to assassinate his father on April 14, 1865, during the same coordinated attack that killed Abraham Lincoln. His father and his brother, Frederick, were severely injured. Gus never married nor did he have children. He passed away on September 11, 1876 in Montrose, NY while on a leave of absence. He is buried with his family in Fort Hill Cemetery in Auburn.