He entered the army as a surgeon, and was attached to the 12th Lancers at the battle of Waterloo, where he treated their commanding officer Frederick Ponsonby.
Here he found it difficult to establish himself in practice, as he held a Scottish degree only, and was therefore not entitled to practise in England.
He accepted the appointment of physician to the Duke of Sutherland, and resided with him for four years, writing on forensic medicine.
A reformer in politics and medicine, Smith was a pioneer of the study of medical jurisprudence, which Robert Christison was trying at the same time to set on a scientific basis.
Smith campaigned, unsuccessfully, to place Scottish and English degrees and licences in medicine on an equal footing.