Joseph Tillie first studied Arts subjects at the University of Edinburgh for a year or two, doing particularly well in political economy, but in 1883 commenced medical studies. He graduated as Bachelor of Medicine (MB) and Master in Surgery (CM) with first class honours in 1886 and was awarded two scholarships and medals in materia medica, pathology, surgery, and midwifery. Continuing his studies in Leipzig and Edinburgh, he submitted a thesis on the pharmacology of curare, curarine, and methyl-strychnium in 1889. The thesis won him a gold medal and the Gunning-Christison Jubilee Prize in the Department of Materia Medica at Edinburgh.
In 1888 Tillie became assistant to Professor T.R. Fraser at the Royal Infirmary, Edinburgh. Three years later he was appointed tutor in clinical medicine, then physician to the Western Dispensary (1893), and in 1895 lecturer on experimental pharmacology at the University of Edinburgh. He published papers on the pharmacology of curare and its alkaloids (1890), the occurrence of haemoglobinuria in blood-pressure experiments (1891), an arrow poison from New Granada (now Colombia) and its botanical source (1893), and a variety of curare acting as a muscle poison (1893). He assisted Professor Fraser in exhaustive research on the active components obtained from the wood of a species of Acocanthera, a tree used in the preparation of arrow poison in East Africa, and was co-author of two papers on this work (1893, 1895). Though he was a capable researcher, he was even better known as a teacher. During 1893 he presented a lecture on "Patent and quack medicines" as part of the Edinburgh Health Lectures, and two years later one on "Heredity and the modern novel" before the Penrith Literary and Scientific Society.
Tillie's health broke down during 1895 and he was granted six months leave from the University of Edinburgh to recuperate in South Africa. He settled in the Free State and took an interest in the professional affairs of the country. In 1897 his name was still listed as a medical practitioner licensed to practice in the Free State, though without an address. At the time of his death in 1898 he resided at Springfontein, in the southern Free State. His widow, Jane L. Barclay, requested the authorities for permission to stay on in her husband's quarters at Springfontein for a while, until she could make other arrangements.