The burning of cattle manure for domestic use, and plant biomass left out in fields, is a common practice in South Asia, specifically Pakistan. According to the 2014 government of Pakistan (GOP) survey, Pakistan had 171 million head of cattle that would produce 345 billion kg of manure, which could easily be converted into 150 billion m3 of biogas. The focus of the present study was to evaluate the benefits from co-digestion of cattle manure (CM) with Dalbergia sissoo leaves (DSL) and Malus domestica leaves (MDL), with a focus on changes in the biodegradability, C/N ratio effect, and synergistic effect. The idea was to adjust the C/N ratio to increase biodegradability at mesophilic range to help the process to produce more methane than 100% manure-based digestion. First, the ideal pH and temperature conditions for mesophilic anaerobic digestion (AD) were optimized to carry out further co-digestion under the same conditions. The results of co-digestion revealed a 40% (251 NmL CH4/g VS) increase in methane yield by replacing 20% of volatile solid in CM-based AD reactors with MDL. This combination also presented a biodegradability of 59% and a synergistic effect (θ) value of 1.40, which corresponded to highly positive synergism reflecting the optimum growth conditions. The DSL/CM co-digestion also followed the same pattern, and the maximum methane yield of 229 NmL CH4/g VS was obtained using a 20/80 DSL/CM combination.