Prior to full-time writing, Southwell worked as a journalist and as Director of Communications for the British Retail Consortium (BRC).^{[citation needed]} In his role at the BRC, he regularly clashed with the British government's media relations department and pressure groups, and played a part in forcing Stephen Byers to scrap the Rip-Off Britain campaign.^{[citation needed]} In a front-page story in The Sunday Times in 2003, Southwell, then a spokesman for the BRC, mentioned that MI5 was coordinating with the business community with regard to potential terrorism.^[1] Southwell later stated in his own books that he had liaised with MI5 on anti-terrorism issues and with the UK Government's emergency planning Cabinet Committee Cabinet Office Briefing Room A (COBRA).^{[citation needed]}

He has acknowledged a specialist knowledge and ongoing interest in the Angry Brigade and conspiracies surrounding the events that inspired VALIS. ^{[citation needed]} In 2005 he was thanked as an inspiration by Gary Russell in his Doctor Who novel Spiral Scratch.

A regular broadcaster in the UK and North America on the subjects of conspiracies and counter-culture, he jokingly claims on the back of his books that if he "dies a mysterious death it will be because he knows too much and has upset some very powerful people".^{[citation needed]}

Southwell lives on a narrowboat on Regent's Canal in London, and married for the second time in 2010.^[2] He wears an eye-patch due to the loss of 80% of the sight in one eye.^[3]

In early 2016 Southwell was a winning contestant on the BBC quiz Pointless.

• Guano Stains Grandma aka Astounding Anagrams (with Anne-Marie Forker) - 1999
• Conspiracy Theories (with Sean Twist) - 1999
• Dirty Cash - 2002
• Conspiracy Files (with Sean Twist) - 2004
• Secrets and Lies - 2005
• Global Gangland The History of Organized Crime - 2006
• 1001 Ridiculous Ways to Die (with Matt Adams) - 2008
• The Kennedy Assassination: The Truth behind the Conspiracy that Killed the President - 2012
• 1001 More Ridiculous Ways to Die - 2013

• ENGLISH DREAMING, ENGLISH RAIN David's blog