His appointment follows an informal review process kicked off by the toy giant last year.
In May, Mattel approached rival media networks concerning its £150 million pan-European media planning and buying account, which is held largely by MindShare. However, a full-scale review was averted and MindShare remains the incumbent in many markets including the UK.
Weller, 43, has a media career spanning over 20 years and for much of this time has concentrated on the children’s and youth market.
As well as leading the global Mattel business, Weller will be expected to use his experience of this market within MindShare Worldwide's dedicated youth insight unit MindShare Play.
Before setting up Generation Media in 2001, Weller was the managing director and a shareholder of the kids advertising specialist First Age Media. Its clients included Lego and Infogames. He is also a previous board director, shareholder and director of strategic planning at The Media Business, which was subsumed into MediaCom.
"It's great to see a large media company putting children's and youth marketing high on its agenda and I am looking forward to harnessing their resource to the maximum in what is a very specialised marketing field," Weller said.
Nick Emery, the chief strategy officer at MindShare Worldwide, added: "Mattel is a key account for MindShare globally and we are delighted that someone of Dean’s ability and rare talent is joining the team."
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