Originally called Easter Cochrane, Johnstone Castle is an altered 16th century L-plan tower house. It consists of a main block of three storeys and a garret, and a wing, remodelled as a massive Gothic tower, which rises a storey higher. A two-storey bartizan crowns the gable of the main block, and there is a massive chimney stack from the original kitchen. The walls are pierced by slits.
The entrance in the re-entrant angle has a porter's lodge. The entrance leads to a vaulted passage, from which the basement chambers are reached. The basement contains the kitchen with a wide-arched fireplace, and the wine cellar with a small stair in the thickness of the walls, climbing to the hall above. The hall on the first floor has been altered.
The property was owned by the Cochranes, who became Earls of Dundonald in 1669, and called their property Easter Cochrane. It passed to the Houstons in 1733 and in 1812 it was remodelled in the Gothic style. Frederick Chopin visited the castle in 1848. The castle was taken over by the local council and most of the mansion was demolished in 1950: only the old part survives. It stands in a housing estate and has recently been restored by a Scottish Castle Association member.