The viaduct was constructed during the first building phase of the railroad, which extended from Baltimore, Maryland to Ellicott's Mills. The Patterson, the third bridge built for the B&O, was similar in construction to the company's first bridge, the nearby Carrollton Viaduct and was named for B&O director William Patterson. It was designed by Caspar Wever and built under the supervision of John McCartney, one of Wever's assistants. McCartney received the contract to build the Thomas Viaduct as a result of his successful completion of the Patterson contract.
The bridge was constructed of granite blocks and was approximately 360 feet (110 m) in length, rising about 43 feet (13 m) above its foundations. It had four graduated arches — two of 55 feet chord (17 m) each and two of 20 feet (6 m) chord each. The smaller arches were introduced for the accommodation of two county roadways, one on each side of the river. The exterior surfaces of the granite blocks were undressed, or rusticated.
In 1830, The viaduct was part of the route used by the B&O's first horse-drawn carriage train to Ellicott's Mills.
The viaduct was almost totally destroyed in an 1868 flood. A single-span Bollman Truss built into the west abutment in 1869 incorporated the original roadway arch and upstream wing wall. The Bollman design was supplanted by another bridge before the railroad was realigned about 400 feet (120 m) upstream in 1902–1903 with the opening of the Ilchester Tunnel.
Today, all that remains at the original crossing is the single masonry roadway arch of the 1829 construction on the west bank and the stone abutment on the east bank, just south of the present railroad bridge. In 2006 a cable-stayed footbridge, with a design that echoes a Bollman Bridge, was added atop the abutments.