Four natural 1,2-naphthoquinones: mansonones C, E, G, and H were isolated from the dichloromethane extract of Mansonia gagei Drumm. heartwoods. Mansonone G was further converted to eight ether and four ester analogues. The structures of mansonones and the analogues were well-confirmed by spectroscopic techniques. All compounds were evaluated for their antibacterial activity against Gram-positive and negative bacteria. Further modification of mansonone G furnished certain analogues displaying better activity than natural mansonones. Particularly, allyl and prenyl ethers of mansonone G exhibited better activity against Staphylococcus aureus with MIC sixty-four times better than their natural compound.