[1][15] After a peripatetic life during the war his parents, both teachers, settled with Rees, an only child, in a rural part of Shropshire near the border with Wales.

His doctoral students have included Roger Blandford,[3][4] Craig Hogan,[5][6] Nick Kaiser[25] Priyamvada Natarajan,[7] and James E. Pringle.

[30] Since the 1990s, Rees has worked on gamma-ray bursts, especially in collaboration with Péter Mészáros,[31] and on how the "cosmic dark ages" ended when the first stars formed.

[32][33] In addition to expansion of his scientific interests, Rees has written and spoken extensively about the problems and challenges of the 21st century, and interfaces between science, ethics, and politics.

[34][35][36][37] In his books Our Final Hour and On the Future, Rees warns that humanity faces significant existential risks in the 21st century due to technological advancements, particularly in bioengineering and artificial intelligence.

[40] Rees thinks the search for extraterrestrial intelligence is worthwhile and has chaired the advisory board for the "Breakthrough Listen" project, a programme of SETI investigations funded by the Russian/US investor Yuri Milner.

[41] In August 2014, Rees was one of 200 public figures who were signatories to a letter to The Guardian expressing their hope that Scotland would vote to remain part of the United Kingdom in September's referendum on that issue.

[44] In his general writings and in the House of Lords, his focus has been on the uses and abuses of advanced technology and on issues such as assisted dying, preservation of dark skies, and reforms to broaden the post-16 and undergraduate curricula in the UK.

Rees has received honorary degrees from a number of universities including Hull, Sussex, Uppsala, Toronto, Durham, Oxford, Cambridge, Harvard, Yale, Melbourne and Sydney.

In June 2022, to celebrate his 80th birthday, Rees was the subject of the BBC programme The Sky at Night, in conversation with Professor Chris Lintott.