Ms McMahon, who is currently deputy editor of handbag.com, was part of its launch team of seven years ago.
Recently she has launched handbag's standalone parenting website gomamatoday.com and was responsible for developing online communities across the handbag titles.
Prior to handbag.com she worked in New Zealand as an advertising copywriter and radio producer.
Neil Sumner, Wallpaper publisher, said: "I am delighted to announce Kay's appointment as digital director of wallpaper.com. Kay's online experience, and passion for the Wallpaper brand, is second to none and I cannot wait for her to start."
Ms McMahon said: "It's an absolute thrill for me to work for a brand I hold in such high regard. I have been reading Wallpaper for 10 years and I couldn't wish for a more perfect role, on any title."
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