Interactions between the body and the environment are dynamically modulated by upcoming sensory information and motor execution. To adapt to this behavioral state-shift, brain activity must also be flexible and possess a large repertoire of brain networks so as to switch them flexibly. Recently, flexible internal brain communications, i.e., brain network flexibility, have come to be recognized as playing a vital role in integrating various sensorimotor information. Therefore, brain network flexibility may be one of the key factors that define sensorimotor skill. However, little is known about how flexible communications within a brain characterizes inter-individual variation of sensorimotor skill and trial-by-trial variability within individuals. To address this, we recruited highly skilled musical performers (i.e. brass instrumentalists) and used a novel approach that combined multichannel-scalp electroencephalography (EEG) recordings, behavioral measurements of musical performance, and mathematical approaches to extract brain network flexibility. We found that brain network flexibility immediately before initiating the performance predicted inter-individual differences in the precision of tone timbre (as represented by spectral centroid of the sound), but not trial-by-trial variability at the individual level. Furthermore, brain network flexibility in broader cortical regions, rather than specific local cortical regions, predicted skilled musical performance, indicating that whole-cortical fluctuations determine individual skill. Our results provide novel evidence that brain network flexibility during movement preparation plays an important role in skilled sensorimotor performance and our findings have potentials for designing a new approach to predict an individual's skill from neural dynamics and a new intervention tool to facilitate physical education.