Born in Glasgow and the son of a portrait painter, McGlashan studied painting at the Glasgow School of Art after national service in the army. He moved to London with the intention of making a living from painting portraits, but was unable to do so. After switching to cartooning and illustrating he curtailed his name to "Glashan".
Glashan's cartoons appeared in a number of magazines and various London newspapers. His original prints often feature fantastical gothic architecture and landscapes with an aspect of the surreal in a striking and distinctive style.