Despite the availability of anticancer drugs, breast cancer remains the most death-causing tumor-related disease in women. Hence, there is a need for discovery and development of efficient alternative drugs, and sources such as plants need to be explored. In this study, antioxidant capacities and inhibitory effects against MCF7 cells of the extracts of stem bark of three Nigerian medicinal plants (Detarium microcarpum, Guiera senegalensis, and Cassia siamea) were investigated. The D. microcarpum extracts had the highest antioxidant and antiproliferative effects, followed by that of G. senegalensis, and the C. siamea extracts had minimal effects. The IC50 values of the methanol and aqueous extracts from the three plants that inhibited the proliferation of MCF7 cells ranged from 78-> 500 μg/ml. Moreover, all the plant extracts but the aqueous extract of Cassia siamea exhibited antimetastatic action and induced apoptosis and cell cycle arrest in MCF7 cells. Liquid chromatography/time-of-flight/mass spectrometry profiling revealed that the five potent extracts contain many phenols and omega-6 fatty acids, and some of the identified compounds (isorhamnetin, eupatorin, alpinumisoflavone, procyanidin B3, syringin, and gallic acid) have been reported to have antiproliferative effects on cancer cells. Hence, the stem bark of these plants could be potential sources of antibreast cancer agents.