He was the eldest son of Paul Siemen Forbes (1808–1886), who travelled to China in 1857 joined his cousin John Murray Forbes who had been heavily involved in China trade and served as partner in Russell & Co., prominent American trading firm in the Far East, and as the United States Consul in Canton.
[2] Like many family members before him, William Forbes joined the Russell & Co. as a partner in 1865 in the trading firm when the American Civil War was about to end.
The Russell & Co. saw the opportunity to introduce American-produced steamboats to the new treaty ports along the Yangtze Basin, as the Qing government underwent a massive industrialisation project.
[4] Russell & Co. refused to join the Hongkong and Shanghai Banking Corporation when was first formed in 1866, as Russells' main rival in shipping, the Augustine Heard & Co. was one of the largest shareholders of the new bank, similar to the Jardines' attitude as its major rival Dent & Co. was on the board and its chairman.
[4] However, the Russell & Co. gradually lost competition against the British Jardine, Matheson & Co. which was more efficient under the leadership of the Chinese comprador Tong King-sing.
Forbes became one of the founders with his brother Henry De Courcy and the Howqua family whom they had established a close relationship for a long time.
[12] Forbes lived with his wife at a villa called Rose Hill on Caine Rd, in Hong Kong.