Built in 1813, Casselman Bridge represents the technical achievements required to complete the first federal highway, the National Road (1811–1818), between Cumberland in western Maryland and the Ohio River during the early national period. Rugged terrain presented a significant challenge to the transport of goods and the early commercial development of this mountainous region. A signature engineering achievement of its day, the Casselman Bridge was the largest single arch stone span in the United States at the time of its completion. The random ashlar stone arch stretches 48 feet between abutments and rises 30 above the waterline. The total span of nearly 360 feet brought the National Road across the Casselman River in Garrett County.
After controversy surrounding federal road building, the National Road was turned over to the relevant state jurisdictions in the mid-1830s. Railroad or canal transport was starting to eclipse the National Road as the most efficient way to move goods over the Appalachian Mountains. By the 1870s the State of Maryland gave the section of the National Road that included the Casselman Bridge to Garrett County. Portions of the old National Road became the right-of-way for U.S. Route 40 in 1925, but the Casselman Bridge was by-passed entirely after 1933. The engineering significance of the bridge was recognized with National Historic Landmark status in 1964. Small sections of the original road are preserved, along with Casselman Bridge, as the focal point of Casselman Bridge State Park.