Johnson was an editor of the journals Democratiya and Engage Journal, the former of which he also helped found. He is a scholar of the labour movement in Iraq, and is a founding member of Labour Friends of Iraq.

A former Trotskyist researching Hal Draper, Johnson is co-author of the Euston Manifesto. Johnson was opposed to the 2003 invasion of Iraq. Since 2003, he has worked with Abdullah Muhsin of the Iraqi Workers Federation. Arguing that the British state should be willing to "get its hands dirty," he proposed that the UK should withdraw from the European Convention on Human Rights, which he argued was obstructing the deportation or "extraordinary rendition" of terrorism suspects "to countries suspected of abusing human rights." Critical of blanket rejections of neoconservatism in foreign policy, Johnson has insisted that "neo-conservatives" deserve "their share of the credit" for "undermining cynical and self-defeating 'realism' and embracing democracy-promotion." Johnson writes frequently for the guardian.co.uk Comment is Free.