The sustainable waste management and energy generation from the lignocellulosic biomass is the predominant methodology for meeting the energy requirement globally. Lignocellulosic biomass of Prosopis juliflora (PJ) can be utilized to produce biomethane with the codigestion of activated sludge, a promising alternative energy source. PJ is a perennial, deciduous shrub and an invasive weed in most of the countries and a source of abundant organic material. Utilization of organic waste in the anaerobic digestion process becomes the need of the hour for environmental protection and for energy generation. In this study, NaOH pretreatment of PJ was performed to remove maximal lignin content, which increases the substrate porosity with least sugar loss. The study of the batch anaerobic digestion of the carbon-rich substrate PJ was conducted at a mesophilic temperature of 37 +/- 2 degrees C with six different feedstock (PJ) proportions. The physicochemical properties of the feedstock revealed the presence of nutrients needed by methanogenic bacteria. The maximum biogas yields from 6 different ratios of feedstocks were 15.0 +/- 4.5, 24.5 +/- 7.3, 45.0 +/- 13.5, 32.5 +/- 9.7, 21.5 +/- 6.4, and 15.5 4.6 mL/g VS for 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100, respectively. The highest cumulative methane yield of 587.3 mL/g VS was seen with the feedstock ratio 60:40 (PJ:SS) as a result of an increase in the microbial activity due to the presence of optimal C/N ratio. Scanning electron microscopy, X-ray diffraction analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis were used to study the structural and biochemical changes of pretreated biomass. Furthermore, the modified Gompertz model fitted with the experimental data and was used to determine the kinetic constants. The correlation coefficient of optimized feed concentration (60:40) R-2 was 0.997.