The honorific title before Newton’s name after his knighthood was “Sir.”
Queen Anne knighted Newton in April 1705 during a royal visit to Trinity College, Cambridge. From then on, his formal name could be styled Sir Isaac Newton. He became only the second scientist to receive a British knighthood, after Francis Bacon.
A knighthood confers the title “Sir” on a man who is knighted individually. It is different from hereditary aristocratic titles such as duke, earl, or lord. Newton was a Knight Bachelor rather than a member of a titled noble family, so “Sir” is the correct honorific rather than “Lord Newton” or “Duke Newton.”
The honor came relatively late in Newton’s career. By 1705 he had already made foundational contributions to mathematics, optics, and mechanics, served as Master of the Royal Mint, and become president of the Royal Society. Historians often note that the knighthood may have had political connections as well as reflecting his public standing.