Born in Melbourne, Victoria, he spent two decades from 1968 to 1988 as an expatriate, first in the US, and then the UK.
Nicholls' early career was as a literary academic, originally with the University of Melbourne. He first travelled to the US in 1968 on a Harkness Fellowship in film making, and has scripted television documentaries. His significant contributions to science fiction scholarship and criticism began in 1971, when he became the first Administrator of the Science Fiction Foundation (UK), a position he held until 1977. He was editor of its journal, Foundation: The Review of Science Fiction, from 1974 to 1978.