Petroleum ether, chloroform and methanol extracts of Ageratina adenophora leaves were screened for assessing bioactive phytochemical constituents. The preliminary phytochemical screening using the standard phytochemical tests detected the presence of alkaloids, steroids, flavonoids, reducing sugars, tannins, saponins, terpenoids, anthraquinones, cardiac glycosides and phenols in different solvent extracts. Total phenolic and flavonoid contents along with total antioxidant capacity of the polar methanolic extract of A. adenophora leaves were evaluated to explore the reliable and potential sources of novel natural antioxidants. The methanol extract leaves revealed a total phenolic content of 30.0 mg gallic acid equivalent/g and 510.0 mg quercetin equivalent/g. The total antioxidant capacity ranges from 16.98 to 94.87% of standard ascorbic acid at concentrations ranging from 0.05 to 1 mg/ml of the plant extract in methanol.