William Thomas (1838 – 21 November 1891) was born in Penzance, Cornwall, and emigrated to the Victorian goldfields in 1851, but returned to England shortly afterwards.
They returned to England, where they purchased the necessary machinery and in 1879 they commissioned R. Honey to erect a flour mill at Leadenhall Street, Port Adelaide, the third in the area.
He was educated at King's School, Port Adelaide, and Prince Alfred College and on leaving found employment as a clerk with the Commercial Bank of South Australia.
After a few years he joined the family flour milling business, and on his father's death was taken into partnership with his uncle.
On the death of W. C. Harrison, one of the partners in another milling firm, Grose & Thomas acquired that business in 1907, and two years later it was formed into a company, carrying on business as W. C. Harrison & Co. His uncle died in 1915 and Henry Thomas became chairman of directors of both companies, a position he held until his death.