Sir William McMillan was an Australian businessman and politician. He was a member of the first federal parliament and served as deputy leader of the Free Trade Party under George Reid, but served only a single term before resigning. He had earlier served in the Parliament of New South Wales, including as Colonial Treasurer, and was prominent in the movement for Federation. He was born in Ireland and became a successful businessman in Australia after moving to Sydney at the age of 18.

The Free Trade Party, officially known as the Australian Free Trade and Liberal Association, also referred to as the Revenue Tariff Party in some states, was an Australian political party, formally organised in 1887 in New South Wales, in time for the 1887 colony election, which the party won. It advocated the abolition of protectionism, especially protective tariffs and other restrictions on trade, arguing that this would create greater prosperity for all. However, many members also advocated use of minimal tariffs for government revenue purposes only. Its most prominent leader was George Reid, who led the Reid government as the fourth Prime Minister of Australia (1904–05). In New South Wales it was succeeded by the Liberal and Reform Association in 1902, and federally by the Anti-Socialist Party in 1906. In 1909, the Anti-Socialist Party merged with the Protectionist Party to form the Commonwealth Liberal Party.