The Priory of Sion, as imagined by fans of the DaVinci Code, never existed and could not be said to have ever had a grand master. The fictional Priory of Sion was based on an almost-equally fictional French society founded, and subsequently dissolved, in 1956 by Pierre Plantard.
Apart from 20th-century hoaxes, a real Priory of Sion did exist during the Middle Ages. According to About.com, this monastic order existed until 1617, when its remaining assets and members were largely absorbed into the larger Jesuit order. Isaac Newton was born in 1643, a full 26 years after the Priory ceased to exist in any form.