The seeds from two mulberry fruits [Morus alba (MA) and Cudrania tricuspidata (CT)] were examined for their oil content, and fatty acid, phytosterol and tocopherol compositions and contents. Moreover, polyphenolic compounds and biological activity of the two defatted seed residue extracts were also evaluated. Oil contents of MA and CT seeds were 29.36% and 16.69%, respectively, while MeOH extracts of the defatted MA and CT seed residues were 5.10% and 6.22%, respectively. The two seed oils were composed of 81.4 and 74.37% linoleic, 5.75 and 11.39% oleic, 8.40 and 10.18% palmitic acid, and 3.52 and 3.0% stearic acids, and two other minor fatty acids, such as linolenic and arachidic acids. MA seed had higher contents of phytosterols (507.59 mg/100 g of oil), tocopherols (99.64 mg/100 g of oil), and total flavonoid (106.50 mg/100 g of seed) than CT seed, whereas CT seed had higher levels of total polyphenol than MA seed. The MeOH extract of MA seed residue showed higher antioxidant, anti-diabetic, and anti-melanogenic activity than that of CT seed residue. trans-Resveratrol (9.62 mg/100 g), quercetin (54.83 mg/100 g), and 4-prenylmoracin (48.70 mg/100 g), were found to be the main polyphenolic components in the MeOH extract of MA seed residue. These results indicate that MA seeds are good sources of essential dietary phytochemicals with antioxidant, anti-diabetic and anti-melanogenic activity.