A cairn or burial chamber was destroyed when the lighthouse was built. Cairns on the Isles of Scilly date back to the Bronze Age and at that time Round Island was probably a peninsula on the northern shore of the main island in the Isles of Scilly. The granite, ashlar, 19 metres (62 ft) tall tower was designed by William Tregarthen Douglass, chief engineer for the Commissioners of Irish Lights and is built on a 35 metres (115 ft) tall mass of Hercynian granite. At the time of building the only access was up a flight of steps cut out of the rock and supplies were taken up the rock face by an aerial hoist. Within the walls of the lighthouse the keepers tended a small vegetable garden, for which the soil was transported to the island.
The light has a focal plane of 180 feet, and originally had an enormous hyperradial optic 4.6 metres (15 ft) high and weighing more than 8 tons. It was built by Chance Brothers & Co of Birmingham and, said at the time, to be ″ .... in relation both to size and character .... the most remarkable works of their kind hitherto achieved.″ A similar optic was installed for the Bishop Rock light the previous year. The optic was replaced in 1966 when the lighthouse was modernised, and again in 1987 when the light was automated. The light now has a 360 millimetres (14 in) optic which emits one white flash every ten seconds, has an intensity of 340,000 candela and a range of 44.5 km (24 nautical miles). The fog signal sounds four blasts every minute. Gusts of winds can be ferocious such as in 1954 when there was continuous gales from 29 November to 16 December. Wind velocities of 177 kph (110 mph) were recorded at the Bishop Rock where seas raced past the window, and on Round Island the wind gauge was destroyed at 177 kph.
Round Island was designated a Site of Special Scientific Interest in 1995 as part of the Pentle Bay, Merrick and Round Islands SSSI. The island is important for its breeding seabirds, especially the European Storm–petrel (Hydrobates pelagicus). Breeding Storm–petrels were unrecorded on Round Island for many years, until one of the lighthouse keepers, mystified by the nightly appearance of black feathers in the living quarters, decided to keep some. When the identity of the bird was discovered, the cat was banished. The Seabird 2000 survey counted 183 occupied nests and a follow–up survey in 2006 found 251 occupied nests on the island; the second highest total in the Isles of Scilly.Puffins (Fratercula arctica) were first recorded as breeding on Round Island in 1850 by Issac North and during the building of the lighthouse it was said that ″They (puffins) were extremly tame and used to walk in and out of the kitchen of the workmen who built the tower. This tameness, and the edibility of their eggs, proved their undoing, for none survive now″. The seabird survey in 2000 also recorded 34 occupied nests of Manx Shearwater (Puffinus puffinus).