Peninnis Lighthouse was one of several prefabricated lighthouses built for Trinity House which were almost all designed by Sir Thomas Matthews.
The unusual tower was built to operate semi-automatically, only requiring a local attendant to draw the curtains open at night and closed in the day to prevent the optical aparatus from catching fire in sunlight, as well as for refuling of the lamp and the winding of the clockwork.
The Lighthouse stands on the southernmost peninsular of the main island of St. Mary's in the archipeligo of the Isles of Scilly, and was a major factor in the placement of the keepers houses on the Garrison, so that the off-duty keepers could still keep an eye on the lighthouse from their homes.
The unique 14 metre high black and white steel tower was built in 1911 and took over from the Lighthouse at St. Agnes, which was then extinguished and retained by Trinity House as a daymark. The tower stands next to a small stone building which housed the fuel tanks for the light, which is connected to the tower by a short pipe.
Part of the unusualy prefabricated design is that only the top portion of the tower is enclosed and the rest is a hollow pile-driven structure made up of 6 cylindrical legs and a central colleum which would have housed the weights that rotated the lights.
Electricity replaced the practice of refueling the light with Acetylene gas in 1992, when the last lighthouse keepers left the scillys and the houses in which the attendants lived when not serving at Bishop Rock or Round Island were sold.
Until late 2011 the light was produced using the original 3rd Order 500 Mm Rotating which was installed when the tower was built - this gave a light visible for up to 17 Nautical Miles, but this was covered up by a plastic sheet and the light was replaced by a simple less interesting LED light on the gallery of the tower. Since this downgrading the light is only visible for 9 Nautical Miles.
The light can be seen and approached from the costal footpath as well as a path from nearby the island's power station.