is a native of Northern Ireland, educated at Portora Royal School, Enniskillen (1960–1968), and a graduate of the University College of Wales, Aberystwyth (B.Sc. in botany; Hons 1st class, 1971) and of the Australian National University, Canberra (PhD. 1976). For 19 years he held the post of horticultural taxonomist in the National Botanic Gardens, Glasnevin, Dublin (1976-1995), and has since 1996 worked as a freelance botanist, author and editor.
While at UCW Aberystwyth, Dr Nelson undertook an ecological study of the unique population of Erica vagans (Cornish heath) in County Fermanagh (published in Botanical journal of the Linnean Society 69: 153–195 (1974)), and after his return from Canberra he was engaged (1976) by the Northern Ireland Conservation Service to survey the vegetation in the Banagher Glen Nature Reserve, County Londonderry. He is an acknowledged expert on the flora of The Burren, County Clare, author of The Burren: a companion to the wild flowers of an Irish limestone wilderness (1991) which is beautifully illustrated by Wendy Walsh, and was commissioned by The Burren Tourism and Environment Initiative (an initiative under the European Regional Development Fund by the Irish Department of Tourism, Sport and Recreation) in 2000 to produce An annotated topographical checklist of the flowering plants, conifers, ferns and fern allies of The Burren Region. His publications include several other fine illustrated books produced in collaboration with Wendy Walsh: these include An Irish florilegium (2 volumes), Trees of Ireland native and naturalized (1993) and A prospect of Irish flowers. He has also written "popular" guides to the flowers of The Burren and the Aran Islands (1999), southwestern Ireland (2001), and Connemara and west Mayo (2001), and more than a hundred research papers on divers botanical and historical subjects.
Dr Nelson holds a number of honorary posts: he is President of the Northern Ireland Heritage Gardens Committee, signalling his expertise as an horticultural historian, Honorary Editor for the Society for the History of Natural History, and International Cultivar Registrar and Honorary Editor for The Heather Society. He is a Fellow of The Linnean Society of London, and a director of The Conservancy of The Burren Ltd.