Sustained unilateral hand clenching alters perceptual processing and affective/motivational state, presumed to reflect increased hemispheric activity contralateral to the side of motor movement. Data from electroencephalographic and imaging studies are contradictory regarding the relationship between sustained hand clenching and brain activity. To investigate relationships between brain activity, sustained unilateral hand clenching, and changes in affect and perceptual processing, frontal hemispheric activity was measured via Functional Near-Infrared Spectroscopy (fNIRS), using derived O2Hb prior to, during, and post-sustained unilateral hand clench. Participants mood and spatial perception were recorded pre- and post-clenching. Sustained unilateral hand clenching altered brain activity and mood, but not spatial perception. O2Hb increased bilaterally following sustained unilateral hand clenching, relative to baseline, regardless of hand. Sustained unilateral hand clenching resulted in greater ipsilateral, compared with contralateral, O2Hb. An interaction between side of hand clench and change in mood was in the direction predicted by theories of hemispheric lateralization of emotion: Following left hand clenching, individuals became more affectively negative, and following right hand clenching, they became more affectively positive. The only relationship between O2Hb and behavioral measures was a positive correlation between left hemisphere O2Hb during hand clenching, and post-clench nervousness.