In this study, a KNO3-loaded coffee husk (CH) ash catalyst was synthesized to produce waste frying oil methyl ester (WFOME) from crude waste frying oil (WFO). Taguchi method optimization was performed to identify the best set of reaction temperature, time, catalyst loading and methanol to WFO molar ratio for maximum WFOME yield. A catalyst composite material (CCM) which is a mixture of CH char and KNO3 (0–65 wt%) was calcined to obtain the catalyst. The KNO3 loading and CCM calcination time effects on the catalyst performance and physicochemical properties were examined. The combustion behavior of the CH and CCMs was investigated using thermal analysis techniques. Utilizing FTIR, XRD, BET, SEM, and pH measurements, the catalyst characterization was carried out. For analytical sample sizes (during thermal analysis) with high KNO3 loading, the thermal reactivity was higher. However, for larger sample sizes (during catalyst synthesis) higher KNO3 loading decreased the CCM global thermal reactivity due to an oxygen permeation resisting ash deposition on the top surface. Thus, KNO3 tuned the catalyst’s physicochemical properties as a function of its loading (optimum 45 wt%) by affecting the combustion characteristics of the CCMs and basic site concentration. The optimum WFOME yield was 97.99 wt% at a reaction temperature of 65 °C, a reaction time of 1.13 h, a catalyst loading of 5.42 wt%, and a methanol to WFO molar ratio of 13.55:1. The WFOME yield was dropped by 46.93 wt% in three consecutive catalyst reuse tests because of active components leaching.