He fills the gap left by the promotion of Mark Collins, who became the managing director of broadcast for WPP's group buying operation Group M last week.
The deputy broadcast director, David Jowett, was widely considered to be the favourite to replace Collins, but in a surprise move, MindShare has raided its sister WPP agency for his successor.
Bignell joins MediaCom after three years as one of the three managing partners of MindShare. Previously, he was a TV buying director at Mediavest.
Nick Lawson, the chief executive of MediaCom, said: "Bignell's appointment demonstrates the advantage of having a media trading arm because it gives people opportunity at three agencies within Group M. He is a top-notch broadcast director and we're really pleased we've got someone at Bignell's level."
Last week, the Group M chief executive, Stephen Allan, announced a string of changes to Group M, which included the former MindShare investment director Nick Theakstone moving to a new position as the chief operating officer.
In other Group M appointments, the MediaCom press director, Steve Goodman, becomes the managing director of print trading, while the managing director of MediaCom Direct, David Kyffin, will perform the same role at the group's direct and digital division.
MindShare said no decision has been taken on Bignell's replacement.
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