Advanced monitoring of neural responses has deepened our understanding of brain functions. However, balancing sampling fidelity and spatial resolution of electrocorticography mapping remains a major challenge in neuroscience and neuroelectronics. Here we describe a flexible, two-dimensional molybdenum disulfide (MoS₂)-based active array for electrocorticography sensing that uses a dual-transistor multiplexed source follower array architecture. We grow wafer-scale trilayer MoS₂ directly on polyimide substrates for a scalable fabrication of active arrays and simultaneously scaling matrices with ultrahigh mapping resolution (up to 51 pixels mm^-2). In vitro recordings of epileptiform activities and sinusoidal potential distributions demonstrate the high spatiotemporal resolution for full-bandwidth electrocorticography recordings using the 50 × 50 MoS₂-based active arrays, which result from the fast on-site multiplexing speeds (τ ≈ 20 ns) and the megahertz response bandwidth of the transducer. In living mice, the active arrays enable high-fidelity electrocorticography monitoring of multiscale neural processes, including auditory-evoked potentials, tonotopic maps and localized multiunit activities at high frequency. Overall, these results suggest that the MoS₂-based active arrays are promising minimally invasive high-fidelity tools for spatiotemporal neuronal monitoring.