William Pinhorne emigrated from England to the Province of New York and was admitted to practice before the New York courts in 1675. He became a freeman of New York City in 1680, an Alderman in 1683, and Speaker of the Assembly in 1685. Upon Governor Sloughter's arrival in the Province in 1691, Pinhorne was appointed to the Governor's Council and served with Nicholas Bayard and Stephen Van Cortlandt on the Committee for Preparing the Prosecution of Jacob Leisler. He was a judge on the special session of the Court of Oyer and Terminer convened to try Jacob Leisler on charges of treason. On May 15, 1691, Pinhorne was appointed an associate judge of the New York Supreme Court of Judicature and a Judge of the Court of Admiralty.
On June 7, 1698, Governor Bellomont ordered Judge Pinhorne stripped of all his offices in New York for harboring a Jesuit in his house. The Governor also revoked the patent to his extensive estate along the Mohawk River in New York. Pinhorne then took up residence at Mount Pinhorne, his estate in New Jersey. He was appointed to the New Jersey Council and to the office of Second Judge of the New Jersey Supreme Court of Judicature. In 1709, Pinhorne became President of the Council and Commander-in-Chief of New Jersey. From the date of his will and the subsequent grant of probate, it appears that he died sometime between May 1719 and April 1720.
Hamlin, Paul M., and Charles E. Baker. Supreme Court of Judicature of the Province of New York, 1691-1704. New York, 1959.