In this paper, a prescribed performance controller based on a command filter is designed for a class of nonlinear systems subject to mismatched disturbances. In the design of intermediate control signal, disturbance estimations are introduced to compensate the effects caused by external disturbances. The so-called “explosion of complexity” problem in traditional backstepping procedure is solved by command filter technique. To enhance the control performance further, a prescribed performance function (PPF) characterizing transient and steady-state performance is used for the tracking error transformation. The proposed controller can ensure that the output tracking error converges to a predefined arbitrarily small residual set, and all the signals of the closed-loop system can be bounded. Theoretical results are validated and illustrated via simulation example.