To investigate the synergism of polyphenol extracts and their mixtures, the total phenolic content, total flavonoid content, and phenol composition of mulberry and sugarcane molasses polyphenols were determined. Additionally, the antioxidant activity of their mixture, which was formulated based on the total phenolic content of each extract, was assessed. Mulberry free polyphenols (MFP) were found to account for 90.3% of the total phenolic content of mulberry molasses (at 677.62 mg gallic acid equivalent (GAE)/100 g), and their total flavonoid content was 736.65 mg rutin equivalent (RE)/100 g. Eight polyphenols were detected, including protocatechuic acid, 3,4-dihydroxyphenylpropionic acid, caffeic acid, syringic acid, p-coumaric acid, ferulic acid, cyanidin 3-glucoside, and rutin. Cyanidin 3-glucoside accounted for 92.6% of the total phenolic content. Sugarcane molasses polyphenols (SMP) had a total phenolic content of 339.69 mg GAE/100 g and total flavonoid content of 314.61 mg RE/100 g. Their polyphenol monomers included gallic acid, protocatechuic acid, chlorogenic acid, caffeic acid, syringic acid, p-coumaric acid, ferulic acid, rutin, and epicatechin. At concentrations of 0~10 mg/mL, SMP, MFP, and their mixtures showed excellent antioxidant activities, with clear dose-response relationships. Further, the SMP+MFP mixture had higher antioxidant activities than either SMP or MFP alone. Furthermore, the combination index (CI) values calculated using the median-effect principle for 0.25, 0.5, 0.75, 1, 1.25, and 1.5 of the IC50 concentrations (for DPPH radical scavenging activity) of the mixture were 0.73, 0.85, 0.94, 0.91, 0.89, and 0.90, respectively. Notably, all CI values were less than 1; therefore, SMP+MFP mixtures exhibited synergism in antioxidant activity.