O'Hanlon was born in 1947 in Dorset, England. He was educated at Marlborough College and then Oxford University. After taking his M.Phil. in nineteenth-century English studies in 1971 he was elected senior scholar, and in 1974 Alistair Horne Research Fellow, at St. Anthony's College, Oxford. He completed his doctoral thesis, Changing scientific concepts of nature in the English novel, 1850–1920, in 1977.
He was elected a member of the Society for the Bibliography of Natural History in 1982, a Fellow of the Royal Geographical Society in 1984 and a Fellow of the Royal Society of Literature in 1993. For fifteen years he was the natural history editor of the Times Literary Supplement.
O'Hanlon has become known for his journeys into some of the most remote jungles of the world, in Borneo, the Amazon basin and Congo. He has also written a harrowing account of a trip to the North Atlantic on a trawler.
Between September 2009 and May 2010, Redmond O'Hanlon was a guest and co-presenter on the programme Beagle: In Darwin's wake for both Canvas in Belgium and VPRO Television in the Netherlands. In the programme, the clipper Stad Amsterdam re-traced the route that Charles Darwin took aboard HMS Beagle (1831–36), a journey that played a seminal role in his thinking on evolution.
He attended the Science & Technology Summit at the World Forum Convention Center in The Hague on 18 November 2010. Fellow Beagle shipmate Sarah Darwin was another featured guest at this convention.
In November 2011 VPRO Television began broadcasting O'Hanlons helden (English: O'Hanlon's heroes). In this eight-part series O'Hanlon introduces the viewer to his heroes of the nineteenth century. The programme was awarded with the prestigious Dutch television award, De Zilveren Nipkowschijf (English: The Silver Nipkow disk). This Silver 1st prize is awarded annually by a professional jury to the best quality television programme. A second eight-part series of O'Hanlons helden was broadcast in the winter of 2013–2014.
- Charles Darwin 1809–1882: A Centennial Commemoration (1982) (contributor)
- Joseph Conrad and Charles Darwin: The Influence of Scientific Thought on Conrad's Fiction (1984)
- Into the Heart of Borneo (1984)
- In Trouble Again: A Journey Between the Orinoco and the Amazon (1988)
- Congo Journey (1996), American edition: No Mercy: A Journey Into the Heart of the Congo (1997)
- Trawler (2005)
- [with Rudy Rotthier] God, Darwin en natuur (2009), English translation: The Fetish Room (2011)
- "Redmond O'Hanlon: Biography". British Council. Retrieved 16 March 2016.
- "Redmond O'Hanlon". Penguin Books. Retrieved 4 October 2014.
- "Beagle website". VPRO. Retrieved 15 March 2010.
- "Meet the Future, Science & Technology Summit 2010". Platform Bèta Techniek (PBT). Retrieved 18 December 2011.
- "O’Hanlons Helden website" (in Dutch). VPRO. Retrieved 18 December 2011.
- "Overzicht O’Hanlons Helden Deel 2" (in Dutch). VPRO. Retrieved 4 October 2014.
- "No Mercy: A Journey Into the Heart of the Congo, Vintage Books". Knopf Doubleday. Retrieved 4 October 2014.