The purpose of this study was to investigate the possibility of using premature mandarin peel powder, a by-product of citrus fruits, for making sponge cakes with the addition of 0%, 3%, 6%, 9%, and 12% of premature mandarin peel powder and analyzing the resultant antioxidative properties and quality characteristics. The study showed that the height, batter yield, and loss rate of sponge cakes decreased as higher amounts of premature mandarin peel powder were added but the weight and specific gravity increased. The moisture content of the sponge cake prepared with 12% premature mandarin peel powder showed the highest values, but the pH was the highest in the control group at 8.12. In terms of color, the L and a values were the highest in the control group, but the b value was highest in the 12% group. There was no significant difference between samples in terms of springiness and cohesiveness, although the study showed a significant increase in the hardness, chewiness, and gumminess as the proportion of premature mandarin peel powder in the sponge cake increased. The total polyphenol content, total flavonoid content, DPPH radical scavenging activity, and ABTS radical scavenging activity all increased significantly as more premature mandarin peel powder was added to the sponge cake. The sensory evaluation results showed that the 6% group had the highest preference in overall acceptability including the color, flavor, taste, and texture. The results of this study proved that the groups with premature mandarin peel powder had better antioxidant activity and sensory properties than the control group and it is therefore judged to have a high potential for use as a food material.