A handheld game controller assembly adapted to combine with a portable electronic device includes a game controller, a receiving receptacle and an external retaining device. The game controller has a pair of holding portions and a transversal portion arranged between the pair of holding portions. The receiving receptacle is fixedly configured at a front end of the transversal portion. The external retaining device has a plugging unit removably engaged in and fixed to the receiving receptacle, and a clipping unit connected to the plugging unit for clipping a portable electronic device.