Sims returned to Ireland, after practicing for a time in Tyrone, he moved to London, where he was admitted a licentiate of the Royal College of Physicians on 30 September 1778.
He was helped by John Coakley Lettsom, and soon acquired a practice.
He served for 22 years as president of the Medical Society of London, and was displaced only by strenuous exertions by younger fellows.
Sims's his inaugural thesis at Leyden was De Temperie Fœminea et Morbis inde oriundis.
Other works are: Sims also completed and corrected Edward Foster's Principles and Practice of Midwifery, 2 vols., London, 1781.