Rennell's first and most influential work was the Bengal Atlas (1779) which was followed by the first detailed map of India (1783), the Geographical System of Herodotus (1800), the Comparative Geography of Western Asia (1831), and important studies on the geography of northern Africa—apparent in introductions to the Travels of Mungo Park and Hornemann. He however forged the geographic data of Park by introducing a mountain range, the Mountains of Kong, supposedly located in the western part of Africa. The fake was intended to support his own theory on the course of the Niger River. The Mountains of Kong remained present in maps until the early 20th century. He also contributed papers to Archaeologia on the site of Babylon, the island of St Paul's shipwreck, and the landing-place of Caesar in Britain. Rennell published a book titled (1788) and dedicated to Sir Joseph Banks.