Previously Southwell worked as a journalist and as Director of Communications for the British Retail Consortium - before turning to full-time writing.
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.
In 2003, in a front page story in The Sunday Times Southwell, then a spokesman for the BRC, mentioned that MI5 was co-ordinating with the business community with regard to potential terrorism. Many people in the conspiracy community believed that this was confirmation that he was allied with Secret Intelligence Service agents and may have been publishing disinformation on behalf of the British security services in his conspiracy books. Southwell later reported in his own books that he had liaised with MI5 on anti-terrorism issues and with the UK Government's emergency planning Cabinet Committee COBRA (Cabinet Office Briefing Room A).
He has acknowledged a specialist knowledge and ongoing interest in the Angry Brigade and conspiracies surrounding the events that inspired VALIS.
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.'
Southwell lives on a narrowboat on the Regent's Canal in London and married, for the second time, in 2010. He wears an eye patch due to the loss of 80% of the sight in one eye.
In early 2016 Southwell Appeared on (and won) the BBC quiz Pointless.