In this study, the individual and combined effects of microwave (MW) and ultrasound (US) processes on the depolymerization of sulfuric acid hydrolysis lignin (SAHL) were investigated in a hybrid microwave–ultrasound chemical reactor. Results revealed that the combined use of MW and US processes led to better results in terms of high yields of bio-oil and monomer production (43.01 and 6.69 wt%), as compared to the individual use of MW (32.83 and 4.71 wt%) and US (21.60 and 3.58 wt%) processes. The highest yield of bio-oil (63.50 wt%), which contained 13.04 wt% monomers, was achieved under the optimized reaction conditions of 180 °C, 80 min, 30% US frequency and a 1,4-dioxane to formic acid ratio of 250 : 50 (v/v). Process parameters such as temperature, time and the content of formic acid added to 1,4-dioxane were chosen carefully and optimized using response surface methodology (RSM). The results of the statistical analysis of variance (ANOVA) showed that the depolymerization temperature and time significantly affected (ρ < 0.05) the yield of bio-oil. In the liquid products, the compounds guaiacol (G1), ethanone, 1-(4-hydroxy-3-methoxyphenyl)-(G3), 2-(4-hydroxy-3-methoxyphenyl)acetaldehyde (G10), 4-ethyl phenol (H1), 4-methyl phenol (H4), and dibutyl phthalate (DBPH) were produced as major monomers. Meanwhile, the GPC results showed that the molecular weight of SAHL (M_w = ∼2156) was significantly reduced to lower molecular weight compounds (M_w = 638). The findings obtained from this study suggest that the applied MW–US-assisted lignin depolymerization process could be a promising alternative to the separate use of the MW or US process and can pave the way to continue advancing the lignin conversion practices as a renewable and sustainable resource of aromatic chemicals. Moreover, the proposed method is believed to possess great potential for conversion of a major waste stream from a pulp industry into aromatic chemical production. Future work will be directed toward the simultaneous use of MW–US-assisted depolymerization of lignin in a continuous flow process mimicking the optimized conditions obtained under batch MW–US conditions.