Mergenthaler joins at a crucial time for Interpublic, as the group seeks to get its finances in order after failing to issue its results for 2004, several months after they were due to be published.
He joins from entertainment marketing firm Columbia House, where he was chief financial officer, a role he took up in 2002. He previously held the role of deputy chief financial officer at Vivendi Universal, and chief accounting officer at drinks giant Seagram. He has also spent 10 years with the accounting firm Price Waterhouse.
Michael Roth, chairman and CEO of Interpublic, said: "His experiences in related industries and at a major accounting firm provide a unique background that will allow him to step in and make significant contributions here at Interpublic.
"I very much look forward to having him as my partner as we progress in the turnaround here at Interpublic."
Interpublic had appointed Mergenthaler last month, but waited until now to name him as the successor to Thompson. Thompson was promoted to the role from that of senior vice-president of finance after Christopher Coughlin departed from Interpublic in July last year.
At the time, Coughlin's departure was put down to him wanting to spend more time with his family. However, he was chief financial officer during a turbulent period, as Interpublic uncovered major accounting irregularities and faced investigation from the US financial regulator.
Mergenthaler said joining Interpublic was a unique opportunity.
"I'm excited to work with Michael, who made it very clear that he was looking for a CFO who would serve as his operating partner and be an important driver of the turnaround to which he is committed," he said.
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