According to a company statement he planned "to continue working but at a less frantic pace".
Frazer's new role will see him responsible for "helping new and existing clients craft successful brand experience activities, around the world".
"Imagination is a place that values talent and know-how above everything," said events director Richard Foulkes.
"Graham has set an industry benchmark on events for world class brands and he's a welcome addition to the Imagination team."
Frazer added: "I have always admired Imagination for its inspired creative ideas, and I cannot think of a company that I am more excited to work for."
Prior to his position at Grass Roots, Frazer was vice-president of Maritz Europa.
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