The suspension bridge, which is a Category A listed building in Scotland,[2] is now the oldest to be still carrying road traffic.

The Berwick and North Durham Turnpike Trust took on responsibility for improving matters by issuing a specification for a bridge.

Brown joined the Navy in 1795, and seeing the need for an improvement on the hemp ropes used, which frequently failed with resulting loss to shipping, he employed blacksmiths to create experimental wrought iron chains.

[6] HMS Penelope was fitted with iron rigging in 1806, and in a test voyage proved successful enough that in 1808, with his cousin Samuel Lenox, he set up a company that would become Brown Lenox & Co.[6] Brown left the Navy in 1812, and in 1813 he built a prototype suspension bridge of 105 feet (32 m) span, using 296 stone (1,880 kg) of iron.

[8] It is not known why Brown became involved with the Union Bridge project, but agreed to take on the work based on a specification dated September 1818.

[8] The bridge proposal received consent in July 1819, with the authority of an Act of Parliament that had been passed in 1802, and construction began on 2 August 1819.

Captain Brown tested the bridge in a curricle towing twelve carts before a crowd of about 700 spectators crossed.

[15] In October 2014, local enthusiasts and activists started a campaign to have the bridge fully restored in time for its bicentenary in 2020.