The appointment follows a pitch against an undisclosed number of non-roster agencies and sees United London join Abbott Mead Vickers BBDO and TBWA\London on the confectionery manufacturer's roster.
A source close to the account said Masterfoods was also looking to retain other agencies involved in the review on a project basis.
The company approached agencies in September with a brief to reclaim the snack brand's former market position. In recent years, it has suffered from the launch of a large number of snacks in the health-bar sector.
Tracker has been without an agency in the UK for the past two years. The brand was previously handled by Grey London as part of its Masterfood Confectionery account, but was dropped from advertising schedules after Grey resigned the business when it was bought by WPP in 2004.