Frith, the 39-year old former editor of Heat, has been acting editorial consultant at the London listings and lifestyle magazine since Thomson's departure.
In addition to his editorial duties, Frith had been asked to find Thomson's replacement.
Time Out London had an average circulation of 67,377 in the second half of 2008, down 22.5 per cent year on year, according to February's ABC figures.
Earlier this year, Time Out Group chief finance officer David King was promoted to chief executive, taking over the day-to-day running from founder and chairman Tony Elliott.
There are now 28 editions of Time Out published under licence or joint ventures around the world, but the flagship magazine in London remains pivotal to group operations. The website timeout.com/london attracts more than 1.7 million unique users per month.
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