Rosneath was a traditional stronghold of the Argylls. The early castle was remodelled in 1630 and destroyed by fire in 1802. The mansion house was built by Bonomi in 1803 and the landscape redesigned at that time. Documentary evidence relies on Roy's map of 1750 and the OS map editions, together with paintings and photographs.
Traditionally, Rosneath was a stronghold of the Argylls and held an important defensive position on the Firth of Clyde. The Castle was enlarged in 1630 by the then Marquis of Argyll, and old paintings show it to be a fortified house. The Castle suffered from a fire in 1802, and in c.1803 the 5th Duke of Argyll commissioned the Italian architect, J. Bonomi, to design a new castle on a site just to the west of the old one. Correspondence between the Duke and Robert Mylne at Inveraray referred to building on the same site as the former castle, after which it was decided to build on a new higher site with views extending to north and south. Bonomi's grand design, described by some as a palace and illustrated by P. Nasmyth in 1809, had two main fronts, both decorated with huge Ionic columns. This exterior decoration was never finished, although the north front was nearly completed and had a very grand portico. The south front featured a balustraded, circular tower, and was to have supported a semi-circular two-storey Ionic-columned portico which apparently was never built. The park and gardens were laid out at about the same time, possibly with the help of Alexander Nasmyth.
By the end of the 19th century Rosneath was a famous landmark, and old photographs show the attractive layout of woods and parks viewed from Helensburgh. At that time it was owned by the 9th Duke of Argyll who had married in 1871 HRH Princess Louise, daughter of Queen Victoria. After Princess Louise's death in 1939, it was left to the Duke of Kent who was killed in action. Ownership of Rosneath then passed to a Mr Woodward who lived in the house for a short period until it was requisitioned for World War II. During the war, Churchill, Eisenhower and Montgomery held talks at Rosneath concerning the D-Day offensive.
By 1954, when the present owners purchased the house and 55 acres of surrounding land for a Caravan Park, the house had already been partially demolished, and in 1961 it was declared dangerous and finally demolished. The farms were sold separately and the woodlands were sold to the Forestry Commission.