He co-authored The Large Scale Structure of Space-Time with University of Cambridge physicist Stephen Hawking, published in 1973, and is considered one of the world's leading theorists in cosmology.

The following year, Ellis co-wrote The Large Scale Structure of Space-Time with Stephen Hawking, debuting at a strategic moment in the development of General Relativity Theory.

In the following year, Ellis returned to South Africa to accept an appointment as professor of applied mathematics at the University of Cape Town, a position he held until his retirement in 2005.

[10] Recently Ellis has also collaborated with Teppo Felin, Denis Noble, and Jan Koenderink on a set of articles published in the journal Genome Biology.

Notable papers include: In 2019 Rhodes University in Grahamstown announced it would award Ellis an honorary doctorate in laws (LLD, hc)[15]