The designed landscape in its present form was laid out between 1750 and 1860. Major improvements to the gardens were undertaken in the late 19th & early 20th centuries and from 1955. James Whitton is thought to have been involved in the design of the terraced garden c.1900.
It is thought that there has been a castle at Finlaystone since the late 14th century when Robert II confirmed the lands on Sir John de Danyelstoun. His son, Sir Robert de Danyelstoun, was keeper of Dumbarton Castle. On his death in 1399, his property was divided between his two daughters; Newark Castle which lies on the shore to the west of Finlaystone went to Elizabeth, whilst Finlaystone itself was inherited by Margaret who married Sir William Cunningham in 1405. Their grandson, Alexander, became the 1st Earl of Glencairn in 1488. His descendant, the 5th or 'Good' Earl, was a strong supporter of the Reformation. In 1556 John Knox gave Communion to the family, reputedly under the yew tree which remains today in the garden. The 9th Earl of Glencairn, William (1610-64), was made High Chancellor of Scotland after the Restoration. The 14th Earl, James (1749-92), was a friend and benefactor of Robert Burns.
In 1746-50 proposals were prepared for alterations to the house but these were not carried out until 1760. On the death of the 15th Earl in 1796, the estate passed to the grandson of the 12th Earl, Robert Graham of Gartmore, in whose family it remained until 1862. In 1830, the railway was constructed through the north of the Finlaystone policies and, as a result, the walled garden was moved from its position near the railway line to its present position. It is thought that some of the existing outbuildings may also have been built around this time. Sir William Cunningham- Graham sold the estate to Sir David Carrick-Buchanan in 1862 who let the house five years later to George Jardine Kidston.
In 1882, Kidston bought the property and began an extensive series of improvements to the house and policies. He commissioned Sir John James Burnett to remodel the house. The gardens were extended, the terraces laid out and the arboretum established. His daughter married Richard Blakiston-Houston and they continued the plantings in the mid-1920s. Their daughter, Marian, married General Sir Gordon MacMillan, and after 1955 further developed the policies and planted extensively in the grounds, particularly after the devastation of the gales of 1968, and also established the nursery in the walled garden. The present owner is their son, George Gordon MacMillan, hereditary chief of the Clan MacMillan. He and his wife, and other members of the family, are actively involved in the upkeep and maintenance of the designed landscape.