Ingram spent 16 years with Saatchis, becoming the chief executive in 1995, the year that the Saatchi brothers left the agency. She took the role of executive chairman in April this year following the arrival of the New Zealander, James Hall, at Charlotte Street.
"It has been a huge decision for me to leave," she said. "But I'm joining an agency with a great future. McCann is in a position to deliver the solutions that clients need in these challenging times."
Hall confessed to "shock" at Ingram's decision to leave. "If you cut one of Tamara's veins then Saatchis' blood would pour out," he said. "It's just one of those occasions when someone phones with the right offer at the right time."
Ingram will take over the running of McCann's London agency and the McCann-Erickson UK and Ireland group from Langdon, who was appointed the agency's regional director for Europe, the Middle East and Africa last week.
Langdon's role as the regional director of Universal McCann passes on to Brian Jacobs, the former managing director of Carat International who joined Universal in May. Chris Shaw, the chairman of Universal in the UK, will report to both Ingram, in her UK group capacity, and Jacobs.
Ingram stressed that she did not plan any management changes at McCann. Chris Hunton will continue as the deputy chief executive of the agency with Nick Wright as the managing director.