Cognitive radio networks (CRN) rely upon dynamic spectrum access that allows spectrum to be used efficiently. The role of the medium access control (MAC) protocol in CRN is to distribute efficiently the spectrum among the users. We propose a MAC protocol for cognitive wireless local area network (WLAN) that allows an unlicensed secondary user (SU) to opportunistically access and transmit data on the licensed channel owned by the primary user (PU). The proposed MAC protocol is an adaptation of IEEE 802.11 distributed coordination function. Extensive performance evaluation of the cognitive WLAN MAC protocol has been carried out as follows. First, a Markov chain model of the cognitive WLAN MAC protocol is developed. By solving our model we derive saturated throughput, packet delay, channel busy probability, conditional collision probability of SU, and interference to PU. Extension of our model for the case of multiple PUs under lightly loaded condition is also carried out. Further, we analyze the energy efficiency of the cognitive WLAN MAC protocol. We calculate the optimal sensing time interval for the SUs and the corresponding sensing energy incurred by the SUs. Effect of imperfect channel sensing on the performance of the proposed protocol is investigated. We validate our model by comparing the analytical results with simulation. © 2018 IEEE.