Adaptive Modulation and Coding (AMC) technique exploits the high frequency diversity in wideband OFDM channels to obtain higher data rates. While prior work has discussed the value of AMC from a theoretical perspective, this work presents the design and performance of a real-time sub-carrier AMC system. The measurements from this AMC scheme implemented on an OFDM transceiver prototype indicate how this design achieves accurate and consistent Signal-to-Noise estimates, which are critical for the success of AMC. The performance of this design is compared with a uniform-adaptation scheme that assigns the same modulation and coding to all sub-carriers. The experiments over the wireless channel shows that for a target coded BER of 10^-5, this technique achieves average data rates of 308.3 Mbps and 237.1 Mbps across a variety of line-of-sight (LOS) and non-LOS locations respectively, which result in 34% and 40% gain over the best uniform-adaptation scheme. Finally, the implementation of this AMC incurs a low overhead of 1.1% of the data rate, and a reasonable complexity, occupying 9.95% of the total transceiver gates on the Field Programmable Gate Array.