Grass silage is an excellent feedstock for biofuel production, however, the recalcitrant cellulosic structure may limit its biodegradability. In this study, the effect of acid pre-treatment with mild thermal treatment conditions on biohydrogen and biomethane production from grass silage was assessed through single-stage (CH4) and two-stage (H2 + CH4) fermentation. Microstructural characterisation showed that pre-treatment significantly reduced the recalcitrance and enlarged the specific area of grass silage. The optimal pre-treatment with 2% H2SO4 at 135 °C for 15 min achieved a total reducing sugar yield of 333.79 mg/g volatile solid (VS) of grass silage. The pre-treated silage led to a hydrogen yield of 68.26 ml/g VS in the first stage hydrogen fermentation, a 3-fold increase compared to untreated silage. The production of volatile fatty acids accordingly increased by 29.2%. In the second stage anaerobic digestion, untreated silage achieved the highest biomethane yield of 392.84 ml/g VS, with a corresponding highest total energy conversion efficiency of 83.5%. Due to a lower biomethane yield, the pre-treated silage presented a decreased total energy efficiency of 68.4%. In comparison, single-stage anaerobic digestion showed lower energy conversion efficiencies of 49.7% and 54.2% for the pre-treated and untreated silage, respectively. Despite the slight decrease in CH4 yield, the pre-treatment led to decreased energy consumption for the operation of anaerobic digestion processes due to the shorter digestion duration.