Incidence of vitamin D deficiency is increasing worldwide. The purpose of this study was to determine if supplementation with vitamin D2 from Portobello mushroom powder would enhance skeletal muscle function and attenuate exercise-induced muscle damage in low vitamin D status high school athletes. Participants were randomised to Portobello mushroom powder (600 IU/d vitamin D2) or placebo for 6 weeks. Participants then completed a 1.5-h exercise session designed to induce skeletal muscle damage. Blood samples and measures of skeletal muscle function were taken pre-supplementation, post-supplementation/pre-exercise and post-exercise. Six weeks supplementation with vitamin D2 increased serum 25(OH)D2 by 9.9-fold and decreased serum 25(OH)D3 by 28%. Changes in skeletal muscle function and circulating markers of skeletal muscle damage did not differ between groups. In conclusion, 600 IU/d vitamin D2 increased 25(OH)D2 with a concomitant decrease in 25(OD)D3, with no effect on muscular function or exercise-induced muscle damage in high school athletes.