Cromwell was born in Glasgow. His mother was a practicing Catholic. He spent his formative years in Barrhead and, mostly, Cumbernauld and graduated from the University of Glasgow with a degree in physics and astronomy. After completing a PhD in solar physics from the same university, which he was awarded in 1987, Cromwell moved to the United States in 1988 to pursue a year-long postdoctoral research at the National Center for Atmospheric Research in Boulder, Colorado.
Returning to Europe, Cromwell joined Shell International in 1989 as an exploration geophysicist. After five months of training in geology, geophysics, and management skills, Cromwell was posted to Shell's exploration and production company in Assen, Netherlands, while living in nearby Groningen. He left Shell in 1993. At that time, he was appointed to a research post at the National Oceanography Centre, Southampton, United Kingdom, but left academia in 2010 to work full-time on Media Lens.
Founded in 2001 by Cromwell and David Edwards, Media Lens is a media analysis website which monitors the broadcast and the print media in the UK, attempting to show evidence of bias, distortions and omissions on such issues as climate change, Iraq and the "war on terror". The founders of Media Lens draw on the 'Propaganda Model' of media control advanced by Edward S. Herman and Noam Chomsky.
Journalist Oliver Kamm, leader writer for The Times, has strongly criticised Media Lens for comments on the Srebrenica massacre and Rwandan genocide, describing the group as a "reliable conduit for denying genocide and whitewashing war crimes". In 2006, Kamm challenged Cromwell's dependence on American historian Howard Zinn, and both men's knowledge of source material relevant to America's atomic bombings of Hiroshima and Nagasaki, asserting that this was "a subject wholly outwith Cromwell's competence". David Cromwell wrote a rebuttal of Kamm's piece on the issue in January 2008. "Not unusually, one has to go to media such as" RT and Press TV "to find any coverage", Cromwell wrote in September 2016 (about the Yemeni civil war), which are "so often bitterly denigrated as 'propaganda' operations by corporate journalists".
Cromwell has written two books with Edwards. The earliest of these titled Guardians of Power: The Myth of the Liberal Media, was published by Pluto Press in 2006. The authors argue, with reference to examples from the press and broadcasting, that the mass media in Britain enable 'state-corporate' power to pursue destructive aims at home and abroad. A later book, Newspeak in the 21st Century, taking a similar approach, appeared in 2009.
As a solo author, Cromwell has written Private Planet (Charlbury: Jon Carpenter Publishing, 2001) and Why Are We the Good Guys?: Reclaiming Your Mind from the Delusions of Propaganda. In a review of the book, Ian Sinclair, writing for the Morning Star, described Cromwell as "one of the most incisive and humane radical writers working today".
Together with historian Mark Levene, Cromwell founded the Crisis Forum, in 2002. According to Paul Robert Bartrop, Steven Leonard Jacobs, it is a "consciousness-raising body that believes humankind is in serious trouble due to an economic and political system that is destroying its ability to sustain its existence." Cromwell and Levene edited a collection of essays, Surviving Climate Change: The Struggle to Avert Global Catastrophe, which was published by Pluto Press in, 2007.
- David Cromwell Why Are We the Good Guys? Reclaiming Your Mind From The Delusions Of Propaganda", Alresford, Hants: Zero Books, 2012, p.8 ISBN 978-1780993652
- Cromwell Why are We the Good Guys, p.5, 7
- "David Cromwell". Zero Books. Retrieved 17 September 2015.
- Cromwell Why Are We the Good Guys, p.28
- Neil Clark "The Left vs. the Liberal Media", The American Conservative, 15 May 2013
- Stated objective of Media Lens Archived 6 February 2008 at the Wayback Machine
- Oliver Kamm "Srebrenica, Trnopolje and the Deniers", TimesOnline, 30 November 2009
- Oliver Kamm "Howard Zinn: Accused of failing to research the claims he makes about Hiroshima", entry from Kamm's blog reproduced on the George Mason University History News Network website, 13 December 2006
- David Cromwell "Racing Towards The Abyss: The U.S. Atomic Bombing of Japan", Media Lens, 15 January 2008. (Last updated 9 April 2013)
- Cromwell, David (13 September 2016). "Menwith Menace: Britain's Complicity In Saudi Arabia's Terror Campaign Against Yemen". Media Lens. Retrieved 29 September 2016.
- For an explanation of this term, see for example Noam Chomsky "The State-Corporate Complex: A Threat to Freedom and Survival", Chomsky..info website, 7 April 2011
- Sally Churchward "Is this the most controversial book of the 21st century?" Southern Daily Echo (Southampton), 28 January 2013
- Ian Sinclair "Why Are We The Good Guys? Reclaiming Your Mind From The Delusions Of Propaganda" Archived 7 March 2016 at the Wayback Machine, Morning Star, 25 November 2012
- Paul Robert Bartrop, Steven Leonard Jacobs Fifty Key Thinkers on the Holocaust and Genocide, Abingdon and New York: Routledge, 2011, p.192
- "Who We Are", Crisis Forum website
- "Climate Change and Violence workshop series 2008 – 2012", Crisis Forum website
- Garry W. Tromby "Book Reviews: Surviving Climate Change", Environmental Conservation, 35:2, June 2008, pp 187–188, 187. This source erroneously substitutes 'Martin' for 'Mark'.