Markhamia tomentosa (Benth.) K. Schum. Ex Engl. (Bignoniaceae) is used traditionally in the treatment of pain, oedema, pulmonary troubles and cancer. The genotoxic and cytotoxic effects of the ethanolic extract of the leaves of M. tomentosa was investigated using the Allium cepa root chromosomal aberration assay. The mitotic index, chromosomal aberrations and mean root length effects of the plant extract on onion bulbs were determined using 50, 100, 200, 400 and 1000 μg/ml concentrations of the plant extract. Tap water was used as the negative control. The result showed a significant (p<0.05) decrease in mitotic index of the onion root tip cells with increased concentration. The plant extract induced stickiness, c-mitosis, bridges and fragments, vagrant, bi-nucleus, multipolar anaphase and laggard chromosomal aberrations in the tested concentrations with no observable chromosome aberrations in the control group. A significant difference (p<0.05) in the mean root length of onions treated with 400 and 1000 μg/ml concentrations and the control group was recorded. Thus, Markhamia tomentosa ethanol extract exerted mito-depressive, genotoxic and cytotoxic effects on Allium cepa root cells