Plectranthus amboinicus Benth (Lamiaceae) is a medicinal plant native to India, and its leaves are widely used in several traditional medicinal preparations. The purpose of this study was to detect and quantify phenolics present in ethyl acetate and acetone extracts of P. amboinicus leaves, and evaluate their antioxidant, antibacterial, antimutagenic and anticancer activities. The HPLC chromatograms of crude leaf extracts indicated the presence of phenolics like caffeic acid, coumaric acid, rutin, quercetin and gallic acid, which were present in the range of 0.01 - 1.41 mg/g in ethyl acetate and 0.03 - 1.93 mg/g in the acetone extract. The acetone extract showed statistically (p < 0.05) higher antioxidant activity ($IC_{50}$, 99.59 ${mu}g/ml$) than ethyl acetate extract ($IC_{50}$, 149.96 ${mu}g/ml$). Statistically (p < 0.05) higher antimutagenicity was shown by acetone extract (46.16%) as compare to ethyl acetate extract (12.16%) at 500 ${mu}g/plate$ concentration. The acetone extract showed higher antibacterial activity than ethyl acetate extract, and both the extracts showed highest activity against B. cereus (375 and 625 ${mu}g/ml$, respectively) and lowest activity against Y. enterocolitica (1000 and 1125 ${mu}g/ml$, respectively). Both the extracts also showed inhibitory effect on cancer cell lines HCT-15 and MCF-7. These results suggest that the leaves of P. amboinicus possess various biological activities, and validate the traditional use of the leaves of P. amboinicus against cold, infection and ulceration.