Understanding the circuit mechanisms underlying the loss of consciousness during general anesthesia is a long-standing question. Activity within cortex drives conscious perception and, hence, the loss of consciousness is thought to result from the disconnection of cortex from subcortex. Here, we identify a change in the correlation structure of neuronal activity during general anesthesia, specific to the primary population of cortical output neurons. We demonstrate that this increased correlation reduces the information output from cortex, suggesting a mechanism for the loss of consciousness. In detail, across different general anesthetics with diverse molecular modes of action, we found that spontaneous activity across the population of layer 5 pyramidal neurons increased in correlation, resulting in an aperiodic alignment of activity (termed “neuronal synchrony”). During transitions to and from anesthesia, the change in synchrony within layer 5 coincides with the loss and recovery of consciousness. Synchronous layer 5 activity extended spatially both within individual cortical areas and between different cortical areas, resulting in a global synchrony across cortex. In contrast, all other cortical cell types, across other cortical layers, did not show a consistent change in synchrony across anesthetics. Layer 5 pyramidal neurons constitute a primary output circuit of cortex. Quantifying the decrease in the variability of activity across layer 5 pyramidal neurons during unconsciousness, we observed a decrease in the information entropy across the population, with the entire population acting as a single effective unit. In contrast, the aperiodic activity of each neuron showed no significant change during anesthesia. Hence, our results show that cortex shifts from a mode characterized by spatially asynchronous outputs transmitting high information, to a mode characterized by spatially synchronous outputs, transmitting low information. This reduction in information output disconnects cortex from sub-cortical structures and, thereby, provides a possible mechanism for the loss of consciousness.