Construction started in 1790; being watched over by the wealthy ironmasters of Merthyr Tydfil, including Richard Crawshay of the Cyfarthfa Ironworks, the canal was thought up as a solution to the issue of transporting the goods (iron ore, coal and limestone) from the valleys to Cardiff, where they would be shipped around the world.

An extension from Merthyr to Crawshay's Cyfarthfa Ironworks was also built, although payment for it resulted in a dispute which was eventually resolved by arbitration.

A plan to build a branch to the Dowlais and Penydarren Ironworks, which would have risen 411 feet (125 m) in only 1.75 miles (2.82 km) was dropped, and was replaced by two tramroads, one from each works.

By this time the project was well over budget, and although the final section to Cardiff was opened on 10 February 1794, it was not well constructed, and there were several stoppages for repairs during 1794.

The canal breached in December, but Dadford refused to start repairs without payment, despite the terms of his contract, and promptly dismissed his workforce and walked away from the job.

3. c. 69), was obtained on 26 April 1796,[1] which enabled the canal to be extended by half a mile (0.8 km), ending in a sea lock in Cardiff docks.

This was a source of dispute for some years, with legal action instituted by both sides and the occasional bout of vandalism to ensure water actually flowed to the Plymouth works.

The canal passed over the Bute Docks Feeder with a small aqueduct built in the late 1830s, at the north-east corner of Cardiff Castle.

The canal was sold to the Marquess of Bute in 1885, who made some improvements at the Cardiff end, but six railway companies were serving Merthyr by 1886, all competing for traffic.

An attempt to evict the sand traders failed when the Ministry of War Transport invoked section 27 and ruled that the sea lock pound must be kept navigable until six months after the present emergency ended.

However, the end came on the night of 5 December 1951, when a steam suction dredger, called Catherine Ethel and weighing 154 tons, crashed into the inner lock gates.

[11] Today, limited traces of the canal remain, about one half being covered by the A470 Cardiff to Merthyr Tydfil trunk road, which was constructed in the 1970s.

[13] The section from Tongwynlais to the Melingriffith Tin Plate Works at Whitchurch has been retained in water and was used for fishing,[14] but is now the Glamorganshire Canal local nature reserve.

[16] A boat weighing machine, one of only four known to have existed on British canals, was originally installed at Tongwynlais and was later moved to North Road, Cardiff.