He was a member of the Supernova Cosmology Project whose leader, Saul Perlmutter, shared the 2011 Nobel Prize for Physics for the team's surprising discovery of the accelerating expansion of the Universe.

At Caltech, Ellis was director of the Palomar Observatory from 2000 to 2005 and played a key role in developing the scientific and technical case, as well as building the partnership, for the Thirty Meter Telescope - a collaborative effort involving Caltech, the University of California, Canada, Japan, China and India destined for Mauna Kea, Hawaii.

He was elected a Fellow of the Royal Society in 1995, appointed a Commander of the Order of the British Empire (CBE) in the 2008 New Year Honours[citation needed], a Fellow and Corresponding Member of the Australian Academy of Science in 2018, and an International Member of the US National Academy of Sciences in 2024.

The story culminates with the prospects for witnessing cosmic dawn - the emergence of the first generation of galaxies from darkness - with NASA's James Webb Space Telescope.

Ellis also contributed the text to a collection of striking photographs by Julian Abrams of the many large telescopes he has used in Modern Observatories of the World.