Monkstone Lighthouse is located in the Bristol Channel, three miles from Flatholm Island. The Monkstone rock is a submerged reef around three miles from Lavernock Point and five miles South of Cardiff. In addition to sandbanks, some of which are exposed at low water, this presents a hazard to shipping in the area.
Monkstone Lighthouse marks the reef and also warns of the nearby hazards.
The first tower was an unlit cast iron beacon, built in 1839. A granite tower, 45 feet high, was built in 1859 to replace the beacon. This tower was strengthened and a circular cast iron tower with a gallery was fitted in 1925. It had an iron lantern and an automatic lantern was operated by acetylene. Iron bands were added to reinforce the stone section of the tower, these bands were painted red.
In 1993 polycarbonate lamps replaced the iron tower and lantern, increasing its height to 75 feet. This also improved the range of the light.