William Forbes (27 Oct 1806-10 Feb 1855) was only nine years old when his father died and his estates were therefore managed by Trustees until he reached his majority at age 25 in 1831. He married Lady Louisa Charteris, daughter of the 8th Earl of Wemyss. William Forbes died in February 1855 after a long illness.

He was a career politician as well as a landowner. He was twice the Conservative MP for Stirlingshire, from 1835 to 1837 and then from 1841 until his death in 1855. He lost his seat in 1837 following accusations of "coercing and intimidating landlords and fictitious votes" [Stirling Observer 3 May 1838].