Webster, who left this week, is to be replaced in the interim by the company's head of sales, James Hanslip, who will take on an expanded remit.
It is understood Webster has left to take a role with a start-up media company, although specific details have yet to be disclosed.
Imagine has yet to decide if it will appoint a long-term replacement for Webster, who has been with the company for about a year.
He joined Imagine Publishing from Emap, where he spent 10 years in a number of roles including head of men's marketing and head of new business.
Imagine Publishing is launching a new rival science title to rival BBC Focus called How It Works.
It will be published monthly from 29 October, backed by a six-figure marketing campaign.
Launch circulation will be 45,000 copies and it will eventually have a print run of up to 75,000 copies.
The magazine's editorial will span space, history and environment, and will feature articles on issues like "freak weather" and "what's a black hole?"
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