Forbes was born in Dublin, the son of Juliette de Guise Forbes, a French woman, and William Forbes, an English railway manager, who was later transferred to London. He had an older brother, Sir William Forbes, who was a railway manager for the London, Brighton, and South Coast Railway.
He was married in the summer of 1889 to fellow painter Elizabeth Armstrong at Newlyn’s St Peter’s Church. Their first home was at the “Cliffs Castle” cottage, which overlooked the sea. They had a son named Alexander (Alec). The couple had a home built for the family in Higher Faughan, Penzance. Elizabeth died in 1912.
In 1915, Forbes married friend and previous student Maudie Palmer, who had been “assistant, helper and friend to the whole Forbes family.” During the First World War his son Alec served in the Duke of Cornwall’s Light Infantry and was killed in August 1916. He is buried in Guillemont Road Cemetery where his headstone bears an inscription composed by his father: HE SAW BEYOND THE FILTH OF BATTLE, AND THOUGHT DEATH A FAIR PRICE TO PAY TO BELONG TO THE COMPANY OF THESE FELLOWS. Stanhope Forbes also sculpted and erected a memorial to his son in their local parish church with the inscription: “I will get me out of my COUNTRY & from my KINDRED & from my FATHER’S house unto a LAND that GOD will shew me”.
Forbes died in Newlyn on 2 March 1947 at the age of 89. He was buried in the churchyard of Sancreed Parish Church