Thomas Telford, a talented Scottish engineer, was commissioned to improve the London to Holyhead road. The port at Holyhead was the start of the sea crossing to Dublin and this route gained importance with the Act of Union of 1800. The journey across Wales was difficult and dangerous. Improving the transport connections with Ireland was important.

By far the greatest part of the work involved was from Shrewsbury to Holyhead. Telford surveyed the route and presented his plans to Parliament in April 1811, and funds were authorised in 1815. Seven turnpike trusts already controlled the route, and Telford negotiated the amalgamation of these to take control of the entire road.

The route from London to Shrewsbury was relatively easy. It was the 106 miles from Shrewsbury to Holyhead that presented the challenge. Telford shaped the route, taking much of the existing path used by the turnpike trusts but reducing the steep grade in places, bridging rivers and generally taking great care to ease the passage of travellers. It is a tribute to his work that the route of the A5 follows the same path almost without change. The design was such that modern heavy trucks and high-speed traffic can travel five times faster than the stage-coaches of his times, without danger or excessive gradients. Telford adopted some of the techniques used by the Romans, layering the roadbed first with large stones, then covering with gravel and paying attention to good drainage.

Finding a way through Snowdonia was hard enough, but a major challenge remained - replacing the ferry across the dangerous Menai Straits between Bangor and the Isle of Anglesey. Strong currents notwithstanding, the Straits had to be kept clear for shipping, meaning that the bridge needed 30 m. (100 ft.) clearance for the masts of sailing ships. Telford's solution was the elegant Menai Bridge, a suspension bridge. Not only did he design the route and arrange the contractors to build it, but he came up with his own design for the elegant milestones on the route, and designed the wrought-iron toll gates and the toll-houses.

Montford Bridge, just west of Shrewsbury, was Thomas Telford's first bridge design. It was built by John Carline Jr and John Tilley between 1790 and 1792. It has three masonry elliptical arch spans, two of 17 m.(55 ft.), and the central one of 18 m.(58 ft.). They are built of red sandstone obtained from Nessliffe Hill four miles distant. The bridge cost £5,800 to build, and was widened in 1963 by the addition of a reinforced concrete slab.