Moving to London in the 1950's, he began as a portrait painter. To support his painting, he dropped the 'Mc' from his name, adopted the psuedonym John Glashan and became a cartoonist and illustrator. His first cartoons appeared in Lilliput in 1959 followed by regular features in Queen magazine and Private Eye from 1961. From the 1960's to the 1990's, his work was published in most newspapers and magazines, many books, and in advertisements for companies such as ICI, Aalders and Marchant and Blue Nun. He was one of the founder members of the British Cartoonist's
Association in 1966.

In 1978 he took over Jules Feiffer's spot on The Observer magazine and began his strip cartoon Genius featuring Anode Enzyme (IQ 12, 794) and his patron Lord Doberman, the richest man in the world. 'Genius' won him the Glen Grant Strip Cartoon award in 1981. This ran until 1983 when he returned to landscape and portrait painting, and from 1988 to 1998 he also contributed weekly cartoons to The Spectator. He exhibited his work at The Francis Kyle Gallery in 1979 and 1983, The Cartoon Gallery in 1991 and at The Fine Arts Society in 1991 and 1994.

John Glashan's cartoon characters ranged from tramps to millionaires to inventors -often tiny figures drawn against a wonderful backdrop of fantastical architecture, beautiful landscapes and ingenious inventions. His unique style evolved from black ink line and wash drawings to exquisite architectural drawings with jewel-like colour, to evocative, subtle watercolour washes depicting landscapes and buildings.

Constantly seeking to extend the boundaries of the cartoon medium, he wrote in 1991: 'A picture, with the addition of writing, as part of the design could form a condensed short story. A series of pictures, varying in size, could be a miniature play. This provided the incentive to try and devise a new method of drawing - to allow a picture I had formed in my mind, to drop, as if by accident, on to the paper. In the way a musician plays an instrument, I would play the drawing.'

He never drew roughs and if not pleased with a picture he had produced, he would start it again. His handwritten captions captured, with a skilful use and play on words, his humorous observations of people and life. He wrote 'I have discovered that the nearer humour approaches seriousness, the funnier it will be. Being funny is not funny. Humour is seriousness in disguise'.