The newly emerging wireless body area networks (WBANs) are intended to support both medical applications and consumer electronic (CE) applications. These two types of applications present diverse service requirements. To satisfy both medical and CE applications with a uniform medium access control (MAC) protocol becomes a new challenge for the WBAN. Addressing this problem, a priority-guaranteed MAC protocol is proposed in this paper. In this protocol, data channels are separated from control channels to support collision-free high data rate communication for CE applications. Priority-specific control channels are adopted to provide priority guarantee to life-critical medical applications. Traffic-specific data channels are deployed to improve resource efficiency and latency performance. Moreover, in order to further minimize energy consumption and access latency, an asynchronous wakeup trigger mode is proposed as an enhancement to the priority traffic. Monte Carlo simulations are carried out for performance evaluation. As compared with IEEE 802.15.4 MAC and its improved versions, the priority-guaranteed MAC demonstrates significant improvements on throughput and energy efficiency with a tolerable penalty on latency performance of bursty traffic in CE applications. Therefore, the customized priority-guaranteed MAC satisfies the service requirements of WBAN by making tradeoff among the performances of different applications.