As a schoolboy, Hagner attended the Georgetown Classical and Mathematical Academy on West Street, St. John's College in Annapolis, Maryland, and received an Artium Baccalaureus degree from Princeton University in 1845 before reading law to enter the bar in 1848.

[2] In 1879, he left Annapolis to return to his birthplace, Washington, D.C., where he was an Associate Justice of the District of Columbia Supreme Court until his retirement in 1903.

Two years later, he acted as Judge Advocate of a Naval Court of Inquiry in the matter of Commander Hunter (who stood accused of exceeding his authority when he accepted the surrender of Alvarado).

In the centennial year, he was a Judge Advocate of the Naval General Court in San Francisco at trial of Pay Inspector Spalding.

The next year, Hagner was on a committee composed of Judge Carmichael, Ross Winans, William Goldsborough, John Contee, and Bowie Davis.

Though he was proposed in 1875 by the Towsontown Journal as candidate for "a good Governor" and spoke out strongly in a speech at the Maryland Institute against fraudulent elections, nothing came of it.

Other cases on his watch were the Potawatomi Indians' land dispute, a challenge to the Constitutionality of the Income Tax Act (1895), a dispute over Frederick Douglass' will (1895–98), Potomac Flats matter determining boundaries of the District of Columbia (he wrote a lengthy opinion on it in 1895, reaching centuries back into English law for precedent), and in 1898, he granted a divorce decree to Frances Hodgson Burnett.

His written works include Memorial of James Clarke Welling, (Records of the Columbia Historical Society, Washington, D.C. Vol.

Born in Richmond, she was the daughter of Heningham Wills and Randolph Harrison of Elk Hill, Goochland County, Virginia.