It is said that the King of Prussia, at an early interview with Baylies, remarked to him that "to have acquired such skill he must have killed a great many people", to which the doctor replied, "Pas tant que votre Majesté (Not as many as Your Majesty)".
After marrying the daughter of Thomas Cooke, a wealthy attorney of Evesham, he began the study of medicine, obtained the degree of M.D.
He issued a pamphlet concerning this quarrel — A Narrative of Facts demonstrating the existence and course of a physical confederacy, made known in the printed letters of Dr. Lucas and Dr. Oliver, 1757, but the controversy ruined Baylies's practice, whereupon he moved to London.
He became licentiate of the College of Physicians in London on 30 September 1765 and made himself notorious by the magnificent entertainments he repeatedly gave at his house in Great George Street, Westminster.
Pecuniary difficulties then forced him to leave England for Germany, where he first settled at Dresden and afterwards at Berlin, where he obtained the post of physician to Frederick the Great.