He was born 13 April 1808, the son of William Mills, a successful merchant trading between American and Britain (with bases in Liverpool and Greenock, who was later Lord Provost of Glasgow.
He studied at Glasgow University, but did not graduate, instead becoming a Manager of the Leith branch of his father's shipping company in 1827 (aged only 19).
He organised steam packets between Leith and London from an office at Newhaven harbour.
Charles Wood dissolved the partnership around 1840 to set up alone, and Mills himself abandoned shipbuilding in 1844 due to a downturn in the market.
In 1869 he set up a newspaper The Northern Star in Aberdeen (not to be confused with its Liverpool predecessor).