Mayhew is taking the non-executive post as the Bridgepoint Capital-backed chain gears up to double its 162-store portfolio in the next three to five years.
Mayhew is a highly regarded executive, with more than 25 years of experience at businesses such as BA and Thomas Cook. He joined John Lewis in 1992 and was managing director for five years before leaving last December.
He said: 'It [Pets at Home] is a really strong business with good shops, products and service. [Founder] Anthony Preston's values are still at the heart of this business and it has a formula that works well.'
His appointment is likely to spark speculation that Pets at Home could be preparing for a Stock Exchange listing. However, Mayhew said: 'It's premature to talk about the next stage or a potential sale or float.'
Last month, he became chairman of 30-store fashion chain Bank. Other non-exec roles are likely to follow as he plans 'up to three or four core activities'.
Bridgepoint bought the£260 million-turnover Pets At Home a year ago in a deal worth£230 million. It refinanced earlier this year after increasing EBITDA by 45 per cent to£34.1 million in the 12 months to March.