α-Lactalbumin (α-La) or β-lactoglobulin (β-Lg) were esterified with methanol. Native proteins and their methylated esters (Est. α-La and Est. β-Lg) were tested for their antibacterial and cytotoxic properties. Esterification extents for Est. α-La and Est. β-Lg were 92% and 88%, with isoelectric points at pH 8 and 10, respectively. The Est. α-La and Est. β-Lg were more inhibiting the growth of pathogenic bacteria compared to the corresponding native proteins. The impact of esterified α-La and β-Lg on MCF-7, A549, and Caco-2 cell lines was assessed and revealed that the concentration inhibiting 50% cellular growth (IC50) for Est. α-La recorded 107.93 µg/mL against MCF-7, 616.44 µg/mL against A-549, and 388.23 µg/mL against Caco2. In parallel, Est. β-Lg achieved IC50 at 101.35, 420.84, and 234.16 µg/mL, against MCF-7, A-549, and Caco2, respectively. Esterified proteins may serve as therapeutic antimicrobial agents against pathogenic bacteria and cytotoxic agents against specific cancer cells.