Lapses in vigilance and slowed reactions due to mental fatigue can increase risk of accidents and injuries and degrade performance. This paper describes a method for rapid, unobtrusive detection of mental fatigue based on changes in electrodermal arousal (EDA), and changes in neuromotor coordination derived from speaking. Twenty-nine Soldiers completed a 2-hour battery of cognitive tasks intended to induce fatigue. Behavioral markers derived from audio and video during speech were acquired before and after the 2hour cognitive load tasks, as was EDA. Exposure to cognitive load produced detectable increases in neuromotor variability in speech and facial measures after load and even after a recovery period. A Gaussian mixture model classifier with crossvalidation and fusion across speech, video, and EDA produced an accuracy of AUC=0.99 in detecting a change in cognitive fatigue relative to a personalized baseline.