In dual-fuel operations, high methane substitution negatively affects combustion performance and creates high hydrocarbon (HC) and carbon monoxide (CO) pollutant emissions. Piston bowl geometry is proven an effective method for improving combustion performance and emissions. In the available study, combustion chamber geometry is experimentally indicated to succeed in dramatically reducing HC and CO pollutant emissions uncompromising nitrogen monoxide (NO) and smoke pollutant emissions. In this study, the effect of piston bowl geometry on the combustion performance and pollutant emissions of a single-cylinder methane-diesel common rail dual-fuel engine was studied experimentally. High-performance piston bowl geometries (Toroidal and Toroidal re-entrant) of conventional diesel operations were used versus original combustion chamber (OCC) geometry in diesel and dual fuel operation for enhancing combustion performance and reducing pollutant emissions. The experiments were conducted at constant 1850 r/min and five different engine loads. The methane energy fraction was set to 50% of the total energy fraction in dual fuel operations. Experimental results showed that the Toroidal re-entrant combustion chamber (TRCC) geometry reduced the long ignition delay period that was due to the methane addition and ensured more stable combustion at all torque conditions. In dual fuel operation, the TRCC geometry improved smoke, HC, and CO pollutant emissions by an average of 18%, 10%, and 3% for all loads, respectively, compared to the OCC geometry. However, NO pollutant emissions were an average of 2.5% higher than OCC geometry for the TRCC geometry. In sum, use of the TRCC geometry is an effective way for providing more complete combustion and reducing emissions under dual fuel operations at all torque conditions between 3 and 9 Nm.