A former IPC executive, Marsden is one of the original team of senior executives who launched the publisher of ShortList and Stylist in 2007. He began as the company’s managing director before becoming chief operating officer last year.
As the director responsible for commercial performance, Marsden is credited with growing the company’s revenues from startup to in excess of £26m last year.
Mike Soutar, the chief executive of Shortlist Media, said: "It has been a great pleasure to work with Karl over the last nine years and his contribution over that time has been epic. We couldn't have built this business without him.
"His commercial skills are second to none, but he has also repeatedly built multi-disciplined high performance teams. Karl has identified, recruited and nurtured many of the stars in the company across the brand, digital and technical teams and he leaves a fine legacy."
The company did not say whether Marsden had another job to go to.
Marsden said: "Epic indeed. The question has to be, who would launch an independent 'Free Media' company into the teeth of the fiercest recession in living memory? Well we did and 2007 seems like yesterday on one hand, and a lifetime ago on the other. What a blast."