Medicinal use of plants is as old as the history of mankind. Medicinal plants have the ability to produce valuable classes of compounds with interesting bioactivities. Due to increasing side effects of synthetic drugs, use of medicinal plants is increasing day by day. Pakistan has about 6000 species of higher plants, of which about 700 species are commonly used for medicinal purposes. Taxonomic, Phytochemical and Biological Screening of Some Selected Medicinal Plants of Lesser Himalaya Pakistan was carried out to determine the accurate taxonomic description, complete phytochemical profile and bioactivity of selected medicinal plants. Four medicinal plants, Spermadictyon suaveolens, Reinwardtia indica, Xylosma longifolium and Celtis eriocarpa were selected based upon ethnomedicinal importance and their least phytochemical exploration. Selected plants were analyzed for taxonomic evaluation, qualitative and quantitative phytochemical screening, proximate analysis, biological activities and LC-MS profiling. R. indica was also analyzed for isolation of phytochemicals. Selected plants were extracted with cold maceration and fractionated in n-Hexane fraction (HF), chloroform fraction (CF), ethyl acetate fraction (EF) and aqueous fraction (AF). Preliminary phytochemical tests revealed the presence of high amount of phenolics and flavonoids in all four plants. R. indica contains high amount of phenolic, flavonoid, tannin, terpenoids and low amounts of alkaloids and saponins. C. eriocarpa contains high amount of phenolic, flavonoid, tannin and terpenoids, while low amount of saponins. S. suaveolens contains high amount of phenolics, flavonoids and tannins. X. 1 2 longifolium contains high amount of phenolics, flavonoids, alkaloids and tannins, while low amount of terpenoids and saponins. Proximate analysis revealed that X. longifolium contains high amount of dry matter (90.30±0.14), low moisture content (9.70±0.14), high fiber, carbohydrates and nutritive values (353.66±0.70). R. indica and X. longifolium showed higher total phenolic contents. High total flavonoid content was found in crude methanolic extract (65.90±1.00) and ethyl acetate fraction (88.01±1.33) of R. indica. Total flavonoid content is also higher in chloroform (74.81±1.58) and ethyl acetate fraction (79.84±0.80) of X. longifolium. R. indica showed higher antioxidant, cytotoxic and antitumor activities in crude methanolic extract, chloroform and ethyl acetate fractions. The highest DPPH EC50 value was exhibited by ethyl acetate fraction (119.64±0.58) followed by crude methanolic extract (209.00±1.53). Chloroform fraction of R. indica also showed higher antitumor IC50 (255.90±0.97), followed by ethyl acetate fraction (297.663±0.91). C. eriocarpa showed higher antioxidant (324.81±2.60), cytotoxic (243.61±1.24) and antitumor (372.76.±0.34) activities only in ethyl acetate fraction. S. suaveolens revealed higher antioxidant activity in ethyl acetate fraction (149.15±0.59) followed by Chloroform fraction (208.13±1.78). X. longifolium showed higher antioxidant activity in crude methanolic extract (107.21±0.16) followed by ethyl acetate fraction (152.96±0.43) and chloroform fraction (187.95±0.26). X. longifolium also showed high cytotoxic and antitumor activity in EF (484.19±1.55 and 235.41±1.51 respectively). UHPLC-DAD-ESI-MS profiling yielded identification of 58 compounds in R. indica, 48 compounds in C. eriocarpa, 50 compounds in S. suaveolens and 60 compounds in X. longifolium. LC-MS profile and bioactivity revealed that bioactivity 3 of R. indica extracts is mainly due to phenolic acids and flavones derivates, C. eriocarpa is due to flavones derivatives, S. suaveolens is due to phenolic acids, Quercetin and luteolin derivatives, while X. longifolium is due to Quercetin, naringenin and apigenin derivatives. UHPLC-MS profile also revealed that S. suaveolens and X. longifolium are rich in Quercetin derivatives. Based upon promising results of bioactivities of R. indica it was selected for compound isolation. Two compounds, Diferuloyl sinapic acid and Esculin were isolated for the first time from R. indica. The present research was very helpful in determining the underlying phytochemical profile and bioactivity correlation of selected medicinal plants.