Background: Polyalthia longifolia is an ornamental plant with various applications in traditional medicine. There is a need to identify its bioactive compounds to justify its medicinal use. Objectives: In this study, the functional groups and bioactive compounds in aqueous methanol extract of P. longifolia leaves and its antimicrobial potential were investigated. Materials and Methods: Bioactive constituents of the extract were assayed with gas chromatography-mass spectrometry (GC-MS) and functional groups by Fourier transform infra-red (FT-IR), while its phytochemical screening and antimicrobial activity were investigated in vitro using standard protocols. Results: Qualitative phytochemical screening showed the presence of therapeutically important phytochemicals, namely, alkaloids, steroidal and cardiac glycosides, saponins, triterpenes, steroids, phytosterols, resins, phenols, flavonoids, tannins, and diterpenes. The extract showed promising bactericidal and fungicidal properties against the test organisms. Staphylococcus aureus was the most resistant of all the test organisms, while other test organisms were significantly inhibited by the extract in a concentration-dependent fashion. FT-IR analysis showed different characteristic peak values confirming the presence of important functional groups, namely, alcohol, alkane, ester, alkene, nitro compounds, amine, carboxylic acid, aromatics, and alkyl halide in the extract. GC-MS analysis identified 25 bioactive compounds in the leave extract. Some of these compounds include 2-methoxy-4-vinylphenol, copaene, aromadendrene, alpha-curcumene, caryophyllene oxide, palmitic acid methyl ester, cis-Z-alpha-Bisabolene epoxide, phytol, alpha-Santoline alcohol, and linolenic acid ethyl ester reportedly possessing antimicrobial, antioxidant, antigenotoxic, antiproliferative, anti-inflammatory, anticarcinogenic, cardio-protective, insecticidal, pesticidal, nematicidal, antiandrogenic, antiviral, and analgesic activities. Conclusion: This study underscores the vast therapeutic importance of P. longifolia leaves as an important source of potentially useful bioactive principles.