Currently deputy editor, Garnett will cover the maternity leave of editor Jennifer Cawthron.
Marina Crook, former editor of the defunct pop title CD:UK, will join Emap Performance's Sneak as assistant editor (news) on 7 March.
Meanwhile, features editor Leo Roberts has been promoted to assistant editor (features). Crook and Roberts will effectively share Garnett's role, with Crook also covering the front of the book. She is currently on holiday and was unavailable for comment.
Sneak, which recorded a weekly circulation of 86,500 at its first ABCs last week, is primarily a gossip magazine for teenagers.
But Emap said this week that PROs should be aware of other elements 'coming to the fore' in the magazine's editorial mix.
Emap Performance MD, Pop, Stephen Palmer, said: 'Style and fashion are becoming more central to the proposition. They are now a much bigger leg of what the magazine is based on than perhaps we first thought.
'Celebrity, gossip and news play a part, of course. But in the same way that Now and even Heat offer more (than gossip to their readers), you need to add to your offering.'
The first ten months of publication have also proved that the age profile of readers is different to what was first envisaged. 'It's slightly older than we thought, with the readers' average age just over 14,' said Palmer.
Sneak hit the news stands last year (PRWeek, 15 March 2002) amid great secrecy, with competitors dubbing it 'Smash Heat' prior to its launch.
Cawthron and Roberts were both hired from Hachette Filipacchi UK monthly Sugar, one of Sneak's main rivals.
In last week's ABCs, Sugar recorded a circulation of 321,200, down 17 per cent year-on-year for July to December 2002. BBC Magazine's It's Hot launched a year ago, recording a circulation of 104,000.