In this study, in vitro antioxidant, antimicrobial, cytotoxic, and cell migration effects of phenolic compounds of Lathyrus tuberosus leaves, known in the Transylvanian ethnomedicine, were investigated. Ultra-high-performance liquid chromatography-tandem mass spectrometry was employed for the analysis of the ethanolic and aqueous extracts. The antimicrobial properties were determined using a conventional microdilution technique. Total antioxidant capacity techniques were used using cell-free methods and cell-based investigations. Cytotoxic effects were conducted on 3T3 mouse fibroblasts and HaCaT human keratinocytes using a multiparametric method, assessing intracellular ATP, total nucleic acid, and protein levels. Cell migration was visualized by phase-contrast microscopy, employing conventional culture inserts to make cell-free areas. Together, 93 polyphenolic and monoterpenoid compounds were characterized, including flavonoid glycosides, lignans, hydroxycinnamic acid, and hydroxybenzoic acid derivatives, as well as iridoids and secoiridoids. The ethanolic extract showed high antioxidant capacity and strong antimicrobial activity against Bacillus subtilis (MIC80 value: 354.37 ± 4.58 µg/mL) and Streptococcus pyogenes (MIC80 value: 488.89 ± 4.75 µg/mL). The abundance of phenolic compounds and the results of biological tests indicate the potential for L. tuberosus to serve as reservoirs of bioactive compounds and to be used in the development of novel nutraceuticals.