His agency and its accounts have subsequently been subsumed into MindShare.
Weller's appointment follows an informal review process kicked off by the toy giant last year. In May, Mattel approached media networks concerning its £150 million pan-European planning and buying account, which is held largely by MindShare.
However, a full-scale review was averted and MindShare remains the agency of record in many markets, including the UK, where the account is worth £32 million. Its brands include Fisher Price Games, Barbie and Matchbox.
As well as leading the global Mattel business, Weller will be expected to use his considerable experience of child and youth marketing 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 child advertising specialist First Age Media.
Weller is also a previous board director, shareholder and director of strategic planning at The Media Business, which was subsumed into MediaCom in 1999, and has spent time working at Grey Advertising.
"This is a great opportunity for me to work on and develop some of the most exciting youth business in the world. It's great to see a large media company putting children's and youth marketing high on it agenda and I am looking forward to harnessing its resource to the maximum in what is a very specialised marketing field," Weller said.
Nick Emery, the chief strategy officer at MindShare Worldwide, said: "Mattel is a key account for MindShare globally."