Sir Ghillean worked from 1963 to 1988 at The New York Botanical Garden as Director of the Institute of Economic Botany and Senior Vice-President for Science. He spent the next 11 years as Director of the Royal Botanic Gardens, Kew, until his retirement in 1999.

He trained as a plant taxonomist and is an expert on the flora of the Amazon rainforests, having led annual expeditions there for over 25 years, collecting more than 450 Amazonian plants new to science. He has been a Fellow of the Linnean Society since 1961 and a Fellow of the Royal Society since 1993.