James Rennell entered the British navy in the beginning of 1756, at the age of 13, with little schooling. He trained himself in navigation and surveying and appears to have drawn his first plan - of the Bay of St Cast, near St Malo, France - in September 1758. (However, surveys before 1764 with which he has been credited are of doubtful validity). In 1762 he transferred to the (British) East India Company as "assistant draughtsman or surveyor", but left the navy the next year after peace had been achieved between Britain and France. In April 1764 he received an appointment as surveyor of the East India Company's dominions in Bengal (now Bangladesh). Here he first surveyed the Ganges and then the Brahmaputra River (to October 1765) and then started a general survey of Bengal. The latter took a number of years and was carried out under very difficult conditions. The journals he kept during 1764-1767 were eventually published in Calcutta in 1910. His work was highly appreciated and in January 1767 his post was upgraded to that of surveyor-general and his rank to that of captain. Meanwhile he had been severely wounded in a skirmish in 1766, an event that affected his health for the rest of his long life. As he now had four assistant surveyors he spent most of his time in Dacca, working on his maps. In October 1772 he married Jane Thackeray and in January 1775 was promoted to major. Two years later he was allowed to retire on pension for health reasons. Returning to England he settled in London to devote himself to the study of geography. His meticulous research led to his recognition as the leading British geographer of his time. He was elected a Fellow of the Royal Society of London in 1781 and at some time also a Fellow of the Royal Society of Edinburgh and a member of several European scientific societies.
In 1780 Rennell published his Bengal atlas, followed by an improved edition the next year. The atlas contained 21 maps and was by far the most advanced publication on Indian cartography at the time. However, his greatest work on Indian geography was probably his Memoir of a map of Hindustan, which was first published in 1783 and expanded over the next ten years. During his later years he produced a number of papers and books on historical geography, including his monumental study The geographical system of Herodotus... (London, 1800). Another major work was A treatise on the comparative geography of western Asia (London, 1831, 2 vols). He also provided the first accurate description of the voyage of the Carthaginian navigator Hanno along the African west coast about 470 BC, and worked out the route followed by the African explorer Mungo Park on his return in 1797.
On his way to England from Bengal in 1777 Rennell noted the currents of the Agulhas Bank, as well as the Agulhas Current to the south. The next year he published A chart of the bank of Lagullas and southern coast of Africa (London, 1778), accompanied by a memoir. His map clearly showed the meandering course of the current along the south-eastern edge of the Bank and then northwards into the South Atlantic. In fact Rennell regarded the Agulhas Current as the link between the Equatorial Currents of the Indian and Atlantic Oceans. He was aware of the great variability of the currents on the Agulhas Bank, and concluded that the flow is strongest in winter. Continuing his investigation on the coastal waters of the Cape in later years he meticulously analysed reports relating to ship drifts in the logbooks of captains that sailed there and produced an accurate description and revised maps of the surface currents around southern Africa. This work was published by his daughter after his death as An investigation of the currents of the Atlantic Ocean and of those which prevail between the Indian Ocean and the Atlantic (London, 1832), a publication that formed the historical basis of ocean circulation studies and made him the pre-eminent pioneer of research on the Agulhas Current. His revised map of the Agulhas Bank showed the inshore counter-current peeling from the main current near Mossel Bay and moving eastwards past Plettenberg Bay. The book was the first work of real scientific merit on the South African ocean environment and made the Agulhas Current one of the best known ocean currents at the time.
Rennell also published papers on the effects of westerly winds in raising the level of the English channel (1809) and the ocean currents along the west coast of North Africa (1821, 1827).