William arrived in China and Hong Kong in 1855, the first of five generations of the Keswick family to be associated with Jardines. He established a Jardine Matheson office in Yokohama, Japan in 1859. He returned to Hong Kong to become a partner of the firm in 1862. He became managing partner (Taipan) from 1874 to 1886. He left Hong Kong in 1886 to work with Matheson & Co. in London as a senior director responsible only to Sir Robert Jardine (1825–1905), a son of David Jardine, William Jardine's older brother and the head of Mathesons in London.