Thornley received her PhD in metric differential geometry from the University of Toronto in 1963. She returned first to Canterbury then took up a two-year lectureship in Trinidad at the University of The West Indies. She then moved back to New Zealand (Nelson and Wellington), combining part-time positions in both academia and the public service (where she worked on economic modelling) with caring for her two young children. In 1989 she was elected first woman President of the
New Zealand Mathematical Society. Thornley presented at the 1990 Conference of the International Mathematics Organisation on the experience of women mathematicians in academia. She also co-authored an article in 2001 on the experience of mathematics doctoral students in New Zealand. The Gillian Thornley Award was inaugurated by the New Zealand Mathematical Society in her honour in 2020. ==Selected works==