Mesh networks have emerged as a strong alternative for providing last mile broadband network access. To further improve their performance, in light of increasing consumer bandwidth demands, much effort is being put into better using the available bandwidth resources. Cognitive dynamic spectrum access is one such technique that allows opportunistic use of underused spectrum, thereby increasing spectrum utilization. In this paper, we present the design and implementation of a new medium access control (MAC) protocol called Dynamic Spectrum Cognitive MAC (DySCO-MAC). It enables cognitive dynamic spectrum access for a single radio, asynchronous wireless mesh/ad hoc network. This protocol is based on a unique approach of node cooperation that takes advantage of the broadcast nature of the wireless channel. We have implemented DySCO-MAC using an open source Linux Wi-Fi device driver and evaluated its performance on a real-world testbed. To the best of our knowledge, this is the first work that implements dynamic spectrum use at the MAC level using COTS Wi-Fi cards. The experimental measurements confirm that cognitive dynamic spectrum access is a promising realistic technology for boosting the performance of next generation wireless networks.