Mental imagery provides an essential simulation tool for remembering the past and planning the future, and its strength affects both cognition and mental health. Research suggests that neural activity spanning prefrontal, parietal, temporal, and visual areas supports the generation of mental images. However, exactly how this network controls the strength of visual imagery remains unknown. Here, brain imaging and transcranial magnetic phosphene data show that lower resting activity and excitability levels in visual cortex (V1-V3), but higher levels in prefrontal cortex, predict stronger sensory imagery. Unlike visual perception, electrically decreasing visual cortex excitability increases imagery strength, while the inverse pattern emerged for prefrontal cortex. These data suggest a neurophysiological mechanism of network cortical excitability that controls the strength of mental images.