Telford began his career as a mason and educated himself to become an architect. In 1786 he was appointed surveyor of public works for Shropshire, a post that entailed the construction of buildings and bridges. Among the spans he built in this era were three over the River Severn, at Montford, Buildwas, and Bewdley, the second being of cast iron.

In 1793 Telford became agent and engineer to the Ellesmere Canal (canals and inland waterways) Company. His two great aqueducts, which carry this canal over the Ceiriog and Dee valleys in Wales at Chirk and Pont Cysylltau, employed a novel use of troughs of cast-iron plates fixed in the masonry. These brought him national fame. Employed in 1803 by the government to assist in the development of the Scottish Highlands, he was responsible for the Caledonian Canal; (Caledonian Canal) harbour works at Aberdeen, Dundee, and elsewhere; and the building of more than 900 miles (1,450 km) of roads, including many bridges. Subsequently, in the course of improving the roads from Chester and Shrewsbury to Holyhead, he built his two famous suspension bridges (suspension bridge) over the River Conwy and the Menai Strait (Wales).

Telford was then employed in improving and building canals to meet the threat of railway competition; this work included a new canal from Wolverhampton to Nantwich and a new tunnel at Harecastle, Staffordshire, on the Trent and Mersey Canal. Among Telford's other works were the St. Katharine Docks, London; roads in the Scottish Lowlands; and the bridges over the Severn at Tewkesbury and Gloucester. He also acted as a consultant for the Göta Canal in Sweden. Telford was the first president of the Institution of Civil Engineers (founded 1818).