Canning joined Edelman in 2001 with a plan to refresh national managements and boost the proportion of the company's non-US revenues from 30 per cent to 50.
As broken last Thursday on www.prweek.com, she is to leave the agency next week and serve out her six-month notice on gardening leave.
Her exit has been prompted by a change in strategic direction for the company, with group chief executive Richard Edelman saying: 'We now recognise that the best way of boosting the amount of non-US business we get is by exporting it from the US. In that respect it is a change of strategy.'
Canning said: 'I'm a change merchant, and you can't be a change merchant unless you are prepared to accept change yourself.'
She is believed to have refused to accept the key change Edelman offered her, of taking on a lesser European role when the global post was abolished.
'That would have been a demotion... we have always had robust discussions about the decisions I've made. I now feel I have created as much value as I can at this company,' Canning said.
The nuts-and-bolts part of her role as acting head of Europe - which she took on in addition to her global responsibilities when she made Milan-based incumbent Rosanna d'Antona redundant - will be taken over by Edelman Ireland general manager Hugh Gillanders.
The company has also hired headhunters to find a new European CEO, to whom all that region's country bosses will now report.
With the international president role abolished, each of the five regions - the US, Canada, Latin America, Europe and Asia-Pacific - will report to the group management in New York. Before this change, the four non-US divisions reported to Canning.