His brother, Joshua Hett Smith was notable as the supposed "dupe" of Benedict Arnold and Major John André when they collaborated during the American Revolution.

He and his brother Joshua Hett Smith escaped prosecution and probable execution by the Commission for Detecting and Defeating Conspiracies in the State of New York in 1778 for the crime of treason due to the memory of their father's influence upon the Justice system: the elder William Smith had, despite the efforts of friends and relatives, refused appointment to the Office of Chief Justice of the Province of New York in 1760, which his son William then accepted.

In 1776, he moved to Albany to avoid the contentious politics but in 1778 declared his allegiance for the loyalist cause and joined the British in New York City.

Smith was escorted across the lines by Aaron Burr and became an important adviser and confidant of the military and civilian officials including both Governor William Tryon and General Sir Henry Clinton.

To which is annexed, A Description of the Country, with a short Account of the Inhabitants, their Trade, Religious and Political State, and the Constitution of the Courts of Justice in the Colony.