Toy manufacturer Mattel has entered into a cooperation agreement with its largest shareholder, EdgePoint Investment, and added one of its partners to its board after investor pressure to explore a sale.

It appointed Lee Matherson, an EdgePoint partner, to its board, effective immediately. EdgePoint currently owns around 18 per cent of Mattel.

Roger Lynch, chairman and chief executive of Mattel, said: “We welcome Lee to Mattel’s Board. Beyond his deep investor and public markets experience, his perspective as a Partner of Mattel’s longstanding and largest shareholder adds a new dimension to our board as we advance our brand-centric strategy and capitalize on the potential of our business.”

The appointment follows reported investor dissatisfaction with the company.

Earlier this month, the Wall Street Journal reported that Ariel Investments, which holds a 5.4 per cent stake in the company, had urged Mattel to explore a sale. The firm said that Mattel’s shares remained significantly undervalued, adding that its progress in performance and profitability had stalled, according to a letter seen by the paper.

In the document, co-chief executive of Ariel John Rogers said: "By our estimates, a strategic buyer would pay a significant premium to your current share price," with the firm adding that other toy companies were likely to find Mattel an appealing proposition, as were entertainment and private equity firms.

The company is also undergoing a wider management change. Its eight-year chief executive, Ynon Kreiz, stepped down last month to become co-chief executive of Skydance Corporation alongside David Ellison. It also replaced its chief financial officer on 20 October, appointing Peter Griffith to replace Paul Ruh, who will serve in an advisory role through to the end of the year.

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