The present study investigates the phytochemical composition of the ethanolic leaf extract of Allium fistulosum L. (Welsh onion) using Gas Chromatography–Mass Spectrometry (GC–MS). The analysis identified 38 bioactive compounds, including fatty acids, esters, sterols, alcohols, phenolics, and other secondary metabolites. Major constituents detected were n-hexadecanoic acid (27.81%), DL-Proline, 5-oxo-, ethyl ester (10.77%), γ-sitosterol (7.94%), 9, 12, 15-octadecatrienoic acid (4.60%), and phytol (4.22%). These compounds are reported to exhibit various pharmacological activity such as antimicrobial, antioxidant, anti-inflammatory, and potential anticancer activities. The phytoconstituents identified highlight the therapeutic relevance of A. fistulosum and provide scientific support for its traditional medicinal applications. The findings further suggest potential utility of this plant in pharmaceutical, nutraceutical, and cosmetic formulations.