The account is moving from Saatchi & Saatchi, which picked up the brief in April 2000, when it was backed by a £20 million spend.
Last year, Monster.co.uk spent around £2 million on advertising. However, it intends to up its adspend next year.
Charles Wasdell, Monster. co.uk's newly appointed head of UK consumer marketing, said: "We have ambitious plans for the year ahead. They require us to invest heavily in marketing activity over the next 12 months and we will have multimillion-pound support across both above-the-line and digital media."
Paul Lawson, the managing director of Leo Burnett, said: "The online recruitment market grew by 60 per cent in the first six months of this year. We are thrilled to be working with the leading brand in such a vibrant market."
Burnett will handle the creative while its digital agency, Arc Interactive, will produce the digital work. ZenithOptimedia will continue to handle media planning and buying.
The new campaign is scheduled to break next year. Until then, the recuitment service will continue to run advertising generated in the US.
Monster.co.uk is one of the UK's leading online recruitment sites. The online re-cruitment market has grown ten-fold since 1999 and is now worth almost £100 million.
Saatchis' last work for the site was a radio ad called "inner voices", which won the July Aerial Award. This was an extension of the agency's last TV campaign.