During his 20-year career as a surgeon-naturalist with the British East India Company, Francis Buchanan (later Hamilton, known in botany as Buchanan-Hamilton and in ichthyology as Hamilton-Buchanan) undertook pioneering survey explorations in several diverse regions of the Indian subcontinent. A naturalist at heart, his collections of plants and animals are often the first from such regions, notably Nepal, Burma (Myanmar) and Bangladesh. Buchanan had wide-ranging interests beyond natural history, using his talent for observation and meticulous recording to amass a huge body of information on the lands and peoples he encountered. However, much of this information remains unpublished in his survey reports, journals and other manuscripts, and so his role in the building of knowledge for these areas has been under-appreciated. Although a keen and able botanist, it is ironic that his multitudinous botanical discoveries are particularly poorly known, with the vast majority of his material on this subject languishing unpublished in archival collections. These include his original records and working notes which show the methods he used when dealing with 'information overload' and arranging his syntheses ready for publication. Notable is his experimentation with Jussieu's Natural System for classifying his Nepalese plants, and his recognition of biogeographic links of the Nepalese flora with Europe and Japan - both ahead of his fellow countrymen in Britain and India. The life of Francis Buchanan awaits the attention of a biographer who can do justice to his many interests, activities and influences. This is the first of two papers covering his life, providing an empirical baseline for future research and correcting misinformation that abounds in the literature. These papers outline Buchanan's professional career, concentrating on his activities in the exploration of natural history, and placing them in the wider context of botanical research in India.