His appointment follows the hiring of Jessica Lovell and Georgia Norton from Mother and R/GA New York respectively into similar roles in April.
Harrison worked on accounts including Telstra, Kraft and VB lager during his time at Droga and has picked up both APG creative planning and Effie effectiveness awards. He has also worked at BMF Sydney and The Monkeys.
Dom Boyd, the head of planning at Adam & Eve/DDB, said: "Toby’s an incredibly exciting addition to the planning team and one of the smartest strategists around. He has a unique knack for pushing ideas into new and unexpected spaces and getting to breakthrough creative solutions, which is exactly what Adam & Eve/DDB is about."