McGuinness, who replaces the retiring Graham Sanderson, will be expected to bring stability to one of the network's prize accounts.
McCann's hold on Unilever has looked weak in recent years - Bartle Bogle Hegarty snatched the frozen-food account in 2003.
McCann also survived a scare recently when Nitro was invited to pitch for one of Unilever's ice-cream brands.
McGuinness, who has been with McCann since 1991, has come to the network's attention as the worldwide account director for MasterCard, one of the network's most successful and well-run pieces of international business.
Dooner said: "Peter has done a superb job of working with MasterCard and with McCann Worldgroup disciplines to ensure the continued power of the 'priceless' campaign domestically, and its flawless and effective expansion to more than 100 markets around the world.
"His experience globally and across disciplines will help ensure that we continue to deliver business-building ideas and executions to Unilever, which is one of our key global clients."
In addition to overseeing the MasterCard business, McGuinness has worked on accounts such as Black & Decker, Johnson & Johnson and AT&T. He was also responsible for spearheading the "healthier living" campaign McCann created for the US Department of Health and Human Services.
At 36, McGuinness is seen by many as one of the network's rising stars and insiders say he is being fast-tracked to a senior management position.
Last year, he was linked with a move to McCann's sister network, Lowe.
McGuinness will be replaced in New York by Richard O'Leary, who has been the chief operating officer of Interpublic's Futurebrand since mid-2002.