As a designer-architect, Clark contributed to the early architecture of St. Louis, including the design of St. Vincent de Paul Roman Catholic Church.

He participated in the Battle of the Sacramento River near Chihuahua City under overall command of Colonel Alexander Doniphan on February 28, 1847.

In early 1861, Clark, strongly pro-secessionist in his political views, was appointed by Governor Claiborne F. Jackson to organize recruits from the Ninth District of Missouri.

He received an appointment as a Brigadier General of the Ninth Division of the Missouri State Guard, but the unit was never formally organized during his tenure, partially because of the very strong pro-Union sentiment in St. Louis.

He was promoted to colonel and assigned to various staff positions; including as Chief of Artillery of the Army of Mississippi under General Braxton Bragg.

During the Appomattox Campaign, he was taken prisoner at the Battle of Sayler's Creek near Amelia Courthouse, Virginia, on April 6, 1865.