Mitchem, who has been working as an advertising consultant since leaving Arnold last year, will manage the marketing across all of the Ogilvy UK companies.
She will work closely with Hugh Baillie, the chief executive of Ogilvy & Mather Advertising, and Annette King, the chief executive of OgilvyOne London.
Mitchem will take on her new role at the end of this month and will report directly to Paul O'Donnell, the chairman of Ogilvy Group UK.
After stints at agencies including Rainey Kelly Campbell Roalfe/Y&R and Leagas Delaney, she joined M&C Saatchi as its head of new business in 2000, before being promoted to group marketing director. Mitchem moved to Lowe as its chief marketing officer in 2005, where she stayed for three years before helping to launch Arnold in the UK.