Previous studies of the mechanical work performed during uphill and downhill walking have neglected the simultaneous positive and negative work performed by the trailing and leading legs during double support. Our goal was to quantify the mechanical work performed by the individual legs across a range of uphill and downhill grades. We hypothesized that 1) with steeper uphill grade, the negative work performed by the leading leg would become negligible and the trailing leg would perform progressively greater positive work to raise the center of mass (CoM), and 2) with steeper downhill grade, the positive work performed by the trailing leg would become negligible and the leading leg would perform progressively greater negative work to lower the CoM. 11 healthy young adults (6M/5F, 71.0 ± 12.3 kg) walked at 1.25 m/s on a dual-belt force-measuring treadmill at seven grades (0, ±3, 6, 9°). We collected three-dimensional ground reaction forces (GRFs) and used the individual limbs method to calculate the mechanical work performed by the individual legs. As hypothesized, the trailing leg performed progressively greater positive work with steeper uphill grade, and the leading leg performed progressively greater negative work with steeper downhill grade (p<0.005). However, unlike during level-ground walking, the leading leg performed considerable positive work when walking uphill, and the trailing leg performed considerable negative work when walking downhill (p<0.005). Our findings reveal important biomechanical aspects of individual leg function during uphill and downhill walking that may help guide the improvement of rehabilitation techniques and prosthetic design.