This paper considers a multi-agent tracking problem for a high-dimensional active leader and variable interconnection topology. The state of the leader not only keeps changing but also may not be measured. To estimate the state such a leader individually, a neighbor-based local controller together with a neighbor-based state-estimation rule is given for each autonomous agent. Then, the authors prove that, with the help of a constructed common Lyapunov function (CLF), each agent can track the active leader with unmeasurable states. Finally, the authors explicitly construct a CLF for an active leader with unknown periodic input for illustration.