In 1861, upon the outbreak of the American Civil War, Agnus enlisted in the 5th New York Volunteer Infantry, and served with merit, rising to brevet brigadier-general before being mustered out of service.

[2] He was educated at College Jolie Clair, near Paris, and, in 1852, set out on a voyage around the world for four years.

When the war ended in 1859, he emigrated from France first to Newport, Rhode Island, and later New York City, where he worked for Tiffany and Company.

[3] In the Peninsula Campaign, Agnus led the charge at Ashland Bridge, and was severely wounded in the shoulder at the Battle of Gaines's Mill.

[7] Captain Agnus was wounded on 27 May 1863, during the siege of Port Hudson, he was promoted to major on 2 September, and for a time had command of his regiment.

He served under General Philip Sheridan, taking part in the battles of Opequon, Fisher's Hill, Winchester, and Cedar Creek.

[1][6] On resuming civil life he was appointed to assistant assessor in the Internal Revenue Service office in Baltimore.

Agnus was appointed US Consul to Derry, Ireland, and confirmed by the Senate, but declined to accept the position.

[12] Black Aggie is the folkloric name given to a statue formerly placed on the grave of Agnus in Druid Ridge Cemetery in Pikesville, Maryland.