His research interests are in the formation and dynamics of galaxies and globular clusters, and particularly in the problem of dark matter in galaxies: he was one of the first to point out (1970) that spiral galaxies contain a large fraction of dark matter. Since then, he has written many papers on dark matter in spiral and elliptical galaxies. He was a founding member of the MACHO collaboration which used microlensing techniques to search for galactic dark matter in the form of compact stellar-mass objects.

For his current research, he uses the optical and radio telescopes in Australia, and also observes with the Hubble Space Telescope and large optical telescopes in Spain, Chile, and Hawaii. He has written about 500 research articles.

Geoff McNamara has been writing about and teaching science and technology since the mid-1980s. He has had approximately 150 articles published in magazines ranging from Electronics Australia, Astronomy, Sky & Space, and Nature Australia.

In 1997 he coauthored a popular level science book "Ripples on a Cosmic Sea - the search for gravitational waves" with Associate Professor David Blair (Allen & Unwin, 1997), and contributeda chapter to "The Universe Revealed" (Mitchell Beazley, 1998).

He taught Ophthalmic Optics at Sydney Institute of Technology from 1987 to 1999, and has presented many courses and talks on astronomy for the public. He has been teaching science at Campbell High School in Canberra since 2000. In 2003 he began teaching Astronomy and the course has continued to grow in popularity. In 2005 the Astronomy courses were completed by approximately 130 students.