We introduce the Masked Humanoid Controller (MHC) for whole-body tracking of target trajectories over arbitrary subsets of humanoid state variables. This enables the realization of whole-body motions from diverse sources such as video, motion capture, and VR, while ensuring balance and robustness against disturbances. The MHC is trained in simulation using a carefully designed curriculum that imitates partially masked motions from a library of behaviors spanning pre-trained policy rollouts, optimized reference trajectories, re-targeted video clips, and human motion capture data. We showcase simulation experiments validating the MHC's ability to execute a wide variety of behavior from partially-specified target motions. Moreover, we also highlight sim-to-real transfer as demonstrated by real-world trials on the Digit humanoid robot. To our knowledge, this is the first instance of a learned controller that can realize whole-body control of a real-world humanoid for such diverse multi-modal targets.