The agency won the business after a three-way pitch and will now oversee the development of all of the charity's DM activity, including recruiting and retaining donors and encouraging people to order legacy gifts.
A pitch was called for the business in November last year, with the process being led by Save the Children's supporter development and innovation director, Susan Davidson.
The previous incumbent on the Save the Children DM account was Whitewater, which was appointed in 2008 to reinvigorate the charity's donor acquisition scheme.
Save the Children has also used Partners Andrews Aldridge, Proximity and EHS 4D for its direct work in the past.
The Good Agency's appointment comes less than a month after Save the Children handed its £4 million advertising business to Adam & Eve after a final shoot-out against Euro RSCG.
Adam & Eve will launch a national mobilisation campaign later this year.