Ingram, the former head of the UK agencies Saatchi & Saatchi and McCann-Erickson, is the new president of the Kantar companies Added Value, Fusion 5 and the Henley Centre.
The chief executives of these three companies - Paul McGown and Charles Broome at Added Value, Dave Moran at Fusion 5 and Sian Davies at the Henley Centre - will report into Ingram, who will in turn report to the Kantar chief executive, Eric Salama.
The appointment comes as WPP announced plans to internationalise the Kantar Group's offering.
Ingram said: "In an ever-increasingly competitive world, those companies that succeed are those that make use of unique insights. I am delighted to be working with three leading-edge organisations that help clients transform their business this way."
Salama added: "Tamara is one of the most talented people in the industry and will be a huge catalyst in taking our companies to another level. She knows what clients need to make best use of work carried out on their behalf and how to build enduring relationships with them."
Ingram has been without a full-time job since she was ousted from McCann-Erickson UK in January after less than 12 months as the chairman and chief executive of the company.
Her departure was sanctioned by McCann's then European chief executive and chairman, Ben Langdon, who was himself sacked from the network in June this year.
Ingram started her agency career with Saatchis in 1985. She spent 16 years with the agency, rising to chief executive in 1995 and executive chairman in 2001.
She is also the chairman of the London Tourist Board and a member of the board of the London Development Agency. In addition, she is the chair of the development board of the Royal Court Theatre and a Trustee of the Beacon Fellowship.