Preston was a born in Edinburgh, on 7 August 1742. His father, also William Preston, was a Writer to the Signet, a form of solicitor. His second, and only surviving child, was encouraged in Classical studies, entering the Royal High School, Edinburgh at six, where he shone in Latin, and would also have studied Greek. He continued his classical studies at college, before becoming secretary to Thomas Ruddiman, a classical scholar whose blindness now necessitated such help. Meanwhile, Preston senior's health and fortunes declined, due to bad investments and supporting the wrong side in the 1745 rebellion. On his death, in 1751, Ruddiman became young William's guardian. He was apprenticed to the printer, Walter Ruddiman, Thomas' brother, but until Thomas' death in 1757 spent most of his time reading to him, and transcribing and copy-editing his work.