William Johnston (1829 – 22 April 1894) was a Scottish-born Australian politician.
He was born at Montrose to stonemason David Mellison Johnston and Agnes Merrillees.
He migrated to New South Wales around 1838 and became a general merchant at Clarence Town on the bank of the Williams River.
In 1877 he was elected to the New South Wales Legislative Assembly for The Williams.
Johnston stood for Durham, but was defeated by Herbert Brown.