Staphylococcus pseudintermedius, Malassezia pachydermatis and Microsporum canis are common opportunistic skin pathogens in animals. This study aimed to evaluate the antimicrobial activities of α-mangostin, α-mangostin-rich extract (AME), rhinacanthin-C, and rhinacanthin-C-rich extract (RRE) against these pathogens and the synergistic antibacterial effects among AME, RRE and ampicillin against S. pseudintermedius. A broth microdilution assay was used to determine the minimum inhibitory concentration (MIC) and minimum bactericidal/ fungicidal concentration (MBC/MFC). Synergistic effects were determined at their respective MICs using the checkerboard and time-kill assays. α-Mangostin, AME, rhinacanthin-C, RRE and vancomycin exhibited strong antibacterial effects against S. pseudintermedius (MICs and MBCs of 0.5-1 and 2-8 µg/mL, respectively), while ampicillin revealed a weak antibacterial activity (MIC of 32 µg/mL and MBC of 64 µg/mL). Combinations of AME and ampicillin as well as RRE and ampicillin exhibited synergistic effects against S. pseudintermedius (FICI of 0.016). Time-kill assays of ampicillin with AME or ampicillin with RRE cocktail confirmed their synergistic effects. In addition, α-mangostin and AME exhibited similar antifungal activity against M. canis and M. pachydermatis (MICs of 1 and 4-8 µg/mL, and MFCs of 4 and 8-16 µg/mL, respectively), while that for rhinacanthin-C and RRE was even more potent (MIC of 0.5 µg/mL, and MFCs of 1-2 µg/mL). Our findings suggest that AME and RRE might be considered as potential antimicrobial agents for treating animal skin infection. They could also be used to overcome the challenge of multidrug resistance by S. pseudintermedius via their synergistic effects with standard antibiotics.