Much of Isherwood's early work was done for Marvel Comics, including such titles as Silver Surfer, Conan the Barbarian, Daredevil, Doctor Strange, and others. Since then, he has been involved in movies as a storyboard artist and a concepts/production illustrator, working with the likes of Bryan Singer, Richard Donner and Darren Aronofsky. He has also painted and designed covers for Canadian publishers, including Ann Diamond'sDead White Males (2000) and Sand for Snow (2003) by Robert Edison Sandiford. In 2011, Isherwood illustrated and prepared the cover for the American novel Antiquitas Lost, by author Robert Louis Smith. A selection of his paintings based on the Greek gods were featured in Issue 59 of the publication Matrix. In 1997, he created LINCOLN-16 (Skarwood Productions), a science fiction saga whose first two issues he wrote, drew, painted, and produced. Geof continues to work in the comic book field on a number of independent projects.
Geof taught advanced comic book, figure drawing and concept art classes and workshops at Syn Studio from 2010 to 2013 an industry focused art school based in Montreal. In 2015, Geof began teaching part-time at Algonquin College in Ottawa, Ontario, in the School Of Media And Design.