Although it is widely agreed that stocking density critically affects the rate of vermicomposting, there is no established stocking density for mixtures of fly ash and other waste materials. This study sought to optimize Eisenia fetida (Savigny, 1826) stocking density for effective biodegradation and nutrient release in a fly ash–cow dung–waste paper (FCP) mixture. Four stocking densities of 0, 12.5, 25, and 37.5 g worms kg−1 were evaluated. Although the 12.5, 25, and 37.5 g worms kg−1 treatments all resulted in a mature vermicompost, stocking densities of 25 and 37.5 g worms kg−1 resulted in faster maturity, higher humification parameters, and a significantly lower final C/N ratio (range 11.1–10.4). The activity of β-glucosidase and fluorescein diacetate hydrolysis enzymes showed faster stabilization at stocking densities of 25 and 37.5 g worms kg−1, indicating compost stability and maturity. Similarly, a stocking density of 25 g worms kg−1 resulted in the highest release of Olsen-extractable P and (NO3 + NO2)–N contents. The 0-, 12.5-, 25-, and 37.5-g treatments resulted in net Olsen P increases of 16.3, 38.9, 61.0, and 53.0%, respectively, after 10 wk. Although compost maturity could be attained at stocking densities of 12.5 g worms kg−1, for faster production of humified and nutrient-rich FCP vermicompost, a stocking density of 25 g worms kg−1 seems most appropriate.