Johnston has written a number of comics, mainly consisting of one-shots and graphic novella. The first consists of parodies, such as Watchmensch and Civil Wardrobe (alluding to Marvel's 2006 story Civil War). The second include his original work, both creator-owned and those based on licensed properties, like Doctor Who: A Room With A Deja View, The Flying Friar (based on the life of Joseph of Cupertino) and Chase Variant which started life at Mam Tor's Event Horizon.
In 2007, he wrote the IDW trading card set "George W. Bush and the Weapons of Mass Distraction." He wrote and drew a number of pages for the Popbitch book and curated the Harrods Comic Timing exhibition of original comic book artwork.
For 2009, he has a story scheduled for the Spearmint anthology from Image Comics with Sleaze Castle writer/artist Terry Wiley. He wrote a short story 'Rustlin Up Business' for the second volume of Outlaw Territory, published in February 2011.
He has also written Kate and William: A Very Public Love Story a comic to commemorate wedding of Prince William and Catherine Middleton, published by Markosia.
In 2012, he wrote a comic serialized in Dark Horse Presents, entitled The Many Murders Of Miss Cranbourne, with art from Simon Rohrmüller. He also wrote a series of parody comics for Boom! Studios, taking on Marvel Studios films, reinterpreted at Iron Muslim, Scienthorlogy, Captain American Idol and The Avengefuls.
Johnston writes and draws weekly cartoons for the UK blogger Guido Fawkes, appearing each Monday and collected at RichAndMark.com.