In 1784, he was appointed superintendent of the royal gardens at Kensington and St James's Palace, a position he kept until his death.

His garden consisted of 40 tons of assorted stone collected from the roadside outside of the Tower of London, some flint and chalk from nearby downland, and some pieces of lava collected from Iceland.

[3] Forsyth created a 'plaister' in 1798 made of lime, dung, ashes, soapsuds, urine, and other various components that was claimed to cure defects in trees and heal "where nothing remained but the bark."

He received a grant of £1,500 from British parliament to continue the creation of the plaister, as the nation was at war in 1799 with Napoleon and needed sound timber to build ships, while the Royal Forests were in poor condition.

[4][5] His great-grandson was the gardener and landscape architect Joseph Forsyth Johnson (1840–1906).