Dore has been named M&S' head of corporate PR on an interim basis. He is on a 13-month contract, covering for Clair Foster who is on maternity leave.
Dore will be responsible for the management of all of the retailer's financial and corporate media and its seven-strong corporate PR team.
He will report directly to Flic Howard-Allen, M&S' director of comms and CSR.
Dore said: 'M&S is one of the most iconic brands in the country and in the press all day, every day on a huge variety of issues. The corporate PR team will continue to manage the brand's reputation on everything from green issues to City matters.'
Dore's fellow MD and co-founder Debbie Wosskow established Mantra in 2000 after leaving Brunswick to set up the firm.
The pair sold Mantra PR to Loewy Group in mid-2007. Wosskow left Loewy in February this year, with Dore departing during the summer.