A nano solid base catalyst (NSBC) was developed from chicken egg shells (CES) and utilized in the cosolvent methanolysis (CSM) and non-solvent methanolysis (NSM) of an equal mix of non-edible oils (waste cooking oil and wild mustard oil) into biodiesel (BD) via the optimized methodology. The NSBC was synthesized by calcinating the raw CES at 850 °C for 2h and a heating rate of 10 °C/min. The NSBC was identified using FESEM, EDX, BET surface area and pore volume, and XRD. The XRD measurement confirmed the transformation of the pristine CES into nano CaO. Also, the NSBC had a BET surface area of 11.47 m2/g, while its average pore diameter was 8.12 nm, demonstrating its mesoporous structure, which is helpful for the methanolysis of lipids into BD. The CSM of mixed oils produced the highest output of BD (91.12 %) using 5.0 wt.% of NSBC, 9:1 methanol: oil molar ratio, 40 wt.% of co-solvent at 60 °C for 1.5 h, while the NSM of the mixed oil produced the highest yield of BD (84.55 %) at 65 °C for 2h using 5.0 wt.% of the NSBC and 12:1 methanol: oil molar ratio. Analysis of the resulting BD employing FTIR spectroscopy and TLC technique confirmed the transformation of the oils mix into BD. Moreover, 1HNMR spectroscopy measurements exhibited that the conversion of mixed oils into BD amounted to 96.96 %. Additionally, the fuel properties of the resulting BD samples conformed to those established for ASTM D6751 standards. In conclusion, the NSBC catalyst derived from the CES could be utilized as an effective catalyst for the methanolysis reaction of lipids. Also, incorporating the co-solvent within the reaction medium effectively reduced the experimental conditions required to achieve the highest transformation of the oils blend into BD compared to the NSM process.