The agency won the account from the incumbent, Chemistry Communications, without a pitch.
TMW, which also works on Unilever brands such as Persil, Vaseline Intensive Care and Comfort, will produce direct and digital campaigns across Slim-fast's range of milkshakes, soups and bars.
The move is a blow to Chemistry, which has been nominated for several awards for its work on the account.
Most recently, its "January Diary" mailer was nominated for best art direction and best copy at the 2007 Campaign Direct Awards. The campaign, which launched in January, comprised a diary containing slimming tips and money-off coupons for products.
Unilever is looking to increase sales of Slim-fast as the shift towards healthy eating and away from strict low-calorie diets has caused a slump in the slimming market.
In the past five years, the overall market value has fallen by 27 per cent from £110.3 million in 2001 to £81 million in 2006, according to figures from Mintel.