Born at Keithmore, Dufftown, James Cantlie's great-uncle was William Marshall, the renowned composer of Scottish fiddle music. He was educated at Milne's Institution before graduating from the University of Aberdeen with an MA in Natural Science in 1871. He continued his studies in London at Charing Cross Medical School where he later became Demonstrator of Anatomy then House Physician, House Surgeon and finally Surgical Registrar. In 1875 he became a Fellow of the Royal College of Surgeons. In 1878 Cantlie became involved in the writing of a manuscript on the instruction of civilian first aid. At Charing Cross Hospital he had recognised a need for the rendering of first aid at the site of an accident. Cantlie himself gave first aid instruction to the Metropolitan Police and was instrumental in the establishment of the Volunteer Medical Staff Corps.