His distinctive line drawing style was perhaps most typically exemplified by his utopian 'Visions' series of posters, commissioned for the Undercurrents' 1974 published anthology Radical Technology. These illustrated scenes of post-revolutionary self-sufficiency in urban and rural settings, and were almost de rigueur decoration for the kitchen wall of any self-respecting radical's commune, squat or bedsit during the 1970s. Of these posters Harper writes:
Funnily enough they were particularly popular in Spain following the death of Franco and the liberalisation that followed that happy event. I think the reason for their success is that although they are utopian images they depict an existence that is immediately approachable -- all it would take is the seizing of a few empty buildings and the knocking down of a few meaningless walls...
Heavily influenced by comic books, Eric Gill and the narrative woodcuts of Frans Masereel, Harper's style evolved in the 1980s into a bolder, more expressionist direction, with much of his later artwork resembling wood or lino cuts, although in fact he still mainly works in pen and ink.
In 1987 Harper's Anarchy, A Graphic Guide, which he both wrote and illustrated, was published by Camden Press. Arguably, this has become a definitive introduction to the subject, combining a thorough overview of anarchism with his distinctive graphic work. As well as being one of Britain's leading radical illustrators, Harper remains a committed and engaged anarchist activist by continuing his involvement with the organisation of the UK's annual Anarchist Bookfair, and small press publishing projects. His work regularly appears in the British newspaper The Guardian.
He has also designed anarchist postage stamps.
In early 2006 Harper survived a massive heart attack.