Lactic acid fermentation of apple pomace was performed using Lactobacillus plantarum EJ2014 to produce gamma-aminobutyric acid (GABA), in the presence of 3% MSG and 0.5% yeast extract. After fermenting for 5 days, apple pomace showed pH and acidity of 6.26 and 0.48%, respectively. Fermentation of apple pomace in the presence of wood ear mushroom (Auricularia auricula-judae) resulted in low acidity (0.16%) and a pH of 6.26. Initially, the number of viable cells was ~1.91×10⁷ CFU/mL, which increased to >3.15×10⁹ CFU/mL after one day. Addition of wood ear mushroom resulted in the highest bacterial stability during lactic acid fermentation for 5 days, resulting in higher number of viable cells (3.86×10⁸ CFU/mL). In particular, GABA production using apple pomace was considerably low; however, it increased following the addition of 2% wood ear mushroom. After 5 days, glutamic acid concentration decreased from 19,552 μg/mL to 1,532 μg/mL, resulting in the production of 13,442 μg/mL GABA. Thus, lactic acid fermentation of apple pomace containing wood ear mushroom resulted in the formation of multifunctional ingredients, including GABA, dietary fibers, and probiotics.