Lear, 33, joins from Clemmow Hornby Inge, where he has been the planning partner since 2003, working on accounts such as Anchor, TalkTalk and Direct Line.
The move reunites Lear with Gav Thompson, CDD's client services director.
Lear and Thompson worked at TBWA in 2002, where Thompson was the managing partner and Lear was the group planning head.
Caspar Thykier, CDD's managing director, said: "Andy is our kind of planner.
He believes in making the complex simple and the simple effective. We are sad to be seeing Alison go, as she has been a real pillar of the agency, but we have a worthy replacement in Andy."
Lear said: "The attraction of CDD is that it is an enormous creative talent. Walt, Sean and Dave are second to none. I can't wait to get stuck in."