There has been a significant focus on developing alternative fuels and energy sources in order to mitigate the adverse effects of fossil fuel combustion on the environment. Among promising options, hydrogen stands out owing to its high energy density and clean combustion characteristics. The use of hydrogen in internal combustion engines, particularly in four-stroke engines, has been extensively studied. However, research on hydrogen combustion in two-stroke engines is limited, even though these engines are widely used in various industries, including the oil and gas sector. This study aims to report novel test results highlighting the exhaust emissions and performance of a large-bore internal combustion reciprocating two-stroke engine fueled with hydrogen-natural gas mixtures, that is the AJAX DPC-81. Natural gas-hydrogen mixtures with different hydrogen concentrations are examined with various engine loads and equivalence ratios. The emission concentrations of nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2), unburned methane (CH4), and volatile organic compounds (VOCs) are measured with corresponding mentioned parameters. Indicated thermal efficiency is computed to expose the combustion performance of the engine. Results found in this study can be adopted for engines that have a similar combustion system to that of the AJAX DPC-81. It is found that emissions are reduced significantly with increasing hydrogen concentration and engine load. The maximum reduction in NOx, CO, CO2, CH4, and VOCs emissions are approximately 90%, 33% 13%, 33%, and 55% respectively at 40% volumetric H2 concentration. Overall, indicated thermal efficiency increases steadily with hydrogen volume fraction in the fuel mixture. At 75% load and 40% volumetric H2 concentration, the engine achieves the highest efficiency of approximately 33%. The optimal operating condition for the AJAX DPC-81 is also determined to be at 60% load with 40% H2 concentration and 60% bypass valve position combined.