Born on 31 December 1813, the son of Honoratus Leigh Thomas, he was educated at the East India College at Haileybury. He went to India in 1832 as a writer in the Bengal service of the company. Poor health affected his career, and he took several absences in England on sick leave; and when Lord Dalhousie offered him in 1852 the post of foreign secretary to the government of India, he declined it.
After acting for a short time as judge at Delhi, Thomas was appointed superintending judge of the Saugor and Nerbudda territory. He retired on a pension in 1857, and spent the rest of his life in scholarly pursuits, attending the meetings of learned societies and writing on Asian archæology. He was elected a Fellow of the Royal Society in 1871, and appointed a Companion of the Order of the Indian Empire in the 1884 Birthday Honours.
Thomas is considered to have made ground-breaking studies in a number of areas of scholarship, such as numismatics (Bactrian, Indo-Scythic, and Sassanian coins); Indian metrology; and Persian gems and inscriptions. His work was recognised by his election as a Fellow of the Royal Society on 8 June 1871, as correspondent of the Institute of France in January 1873, and as honorary member of the Russian Academy; and by his decoration as Companion of the Indian Empire. His major works were:
Other publications included:
Many of his numerous short papers appeared in the Numismatic Chronicle between 1847 and 1883. Rather more were in the Journal of the Royal Asiatic Society, of which he was a member for 40 years and treasurer for 25.