In 1912, the Marietta City Board of Education informed the Library that it would have to move to a different location because of the size of the high school. The trustees purchased an option on a property at Fourth and Putnam Street that fall, but the lot was covered in water due to a flood in 1913. The library trustees then chose an elevated site called Capitolium near Fifth and Washington Streets. The city of Marietta gave the property to the Library Association on May 20th, 1915 to construct a free public library. It was set aside by the Ohio Land Company in January of 1796 for public buildings. Andrew Carnegie pledged $30,000 for library construction while the city provided the lot and monies for operation. Public donations yielded $4,000. Construction began during the summer of 1916 and was completed in 1918. Today, the 615 Fifth Street location serves as the main library for the Washington County Public Library System.
The library sits on a temple mound called the Capitolium. It is eight feet high, fifty feet long and twenty feet wide. When excavating, no bones were found to indicate that it was a burial place. Capitolium also was part of the earthworks systems that also include Quadranaou at Sacra Via.