Germination is one of the most important techniques to enhance the nutrients and functionality of legume seeds. This academic work investigated the effects of pre-treatment with acidifying soaking water and germination time on changes in concentrations of gamma-aminobutyric acid (GABA) and nitrogenous constituents, and protein-related enzyme activities in mung bean seeds. Higher amounts of crude protein, non-protein nitrogenous, free amino acids, and enzyme activities, but lower protein nitrogenous concentrations were found when soaking in more acidic water and germinating at longer germination time. Steeping water of pH 5.5 and germination duration of 8 h induced mung bean seeds to manifest the highest activities of glutamate decarboxylase (GAD) (60.9 U/g powder, db) and protease (2.81 U/g powder, db), responsible for the highest values of GABA (1.60 g/kg, db), free amino acids (5.92 g/kg, db) and non-protein nitrogen (14.7 g/kg, db), and the lowest amount of protein nitrogen (30.8 g/kg, db). These findings indicate that pre-treatment with acidic soaking water before germinating was more likely to enrich the GABA and amino acid-containing compounds of the mung bean seeds.