[5] Following his master's degree, Mohrhardt began his career as assistant librarian and faculty member at Colorado State College of Education in 1933.

[2] During this time he traveled throughout the United States meeting various library and education representatives of junior college with the purpose of compiling a list of books beneficial to those institutions.

[2] During World War II, the Library of Congress was interested in protecting some of its more valuable collections and Mohrhardt offered surplus space available at Washington and Lee University for this purpose.

During the war Mohrhardt performed military service in the U.S. Army Air Force, U.S. Navy and was involved in civilian duty.

During this time he moved often, was stationed at Fort Lee for army service in 1942, then went to Indianapolis as a civilian instructor in electronics (during his education Mohrhardt received some training from General Electric)[2] and aircraft turrets; in 1943 and 1944 he was involved in radar work for the Navy.

[2] In 1946, he began as assistant and was then promoted to chief of the Library and Report Division of the Office of Technical Services at the Department of Commerce.

Processing involved indexing and listing of the materials, and sometimes translations; the documents were then appraised for their value to the public and private sector and made widely available, where appropriate.

Due to the volume of materials, it encouraged the development and application of mechanization as well as new automated techniques for information handling[2] In 1947 Mohrhardt would leave OTS to work as a contact consultant at Brookhaven National Laboratory of the Atomic Energy Commission.

During his time there he developed a central acquisitions and cataloging system that increased the direct services the center provided to the VA libraries around the country.

He established a reputation for skillfully organizing and streamlining the procurement and cataloging systems that endeared him for assignment to the U.S. Department of Agriculture Library.

In 1958 he represented the Department of Agriculture in U.S. Senate hearings on the Science and Technology Act of 1958 and spoke on the cooperation among the national libraries.

In 1955 Mohrhardt would help found the International Association of Agricultural Librarians and Documentalists (IAALD); he would serve as first president and hold the position for three terms - spanning almost 15 years.

[11] He worked extensively in cooperation with the National Diet Library and led multiple scientific and library-related activities in Japan.

He saw the importance of global information sharing and used his career to foster relationships within the US government and abroad to encourage and reflect that belief.