Edward Clark was Thomas U. Walter's student, chief assistant, and successor. Prior to working with Walter, he had received training in freehand and mechanical drawing as well as engineering. Clark was the superintendent of construction on the Patent Office and Post Office additions under Walter. He belonged to the American Institute of Architects.

Clark was appointed Capitol architect upon his mentor's resignation and completed the extension project in 1868. A year earlier, the old Capitol and the Grounds, as well as the extensions, were placed under the control of the Architect when the office of Commissioner of Public Buildings was abolished. Clark introduced many technological improvements to the Capitol, including electricity, steam heat, and elevators. During Clark's tenure, the Capitol Grounds were greatly enlarged and Frederick Law Olmstead, the greatest landscape architect of the day, was commissioned to design the grounds and terraces. Also during his administration, the Library of Congress moved to its own building, and the west central interior of the Capitol was reconstructed.