With the aim of CO2 emissions reduction in the maritime sector, dual fuel engines operating on natural gas are the most prominent technical and commercially available solution. A promising variant is the two-stroke high-pressure natural gas injection engine, utilizing diesel pilot fuel injection for ignition of the gaseous fuel while being able to operate in diesel-only mode. In this study, a comparative analysis of the performance and the combustion mechanism of dual fuel and diesel mode for this engine type is conducted using experimental data. Studies based on measurements conducted on actual scale are limited in the literature due to the engines’ sheer size not allowing lab testing. The analysis was conducted using measurements acquired during the factory acceptance tests involving conventional operating data and cylinder pressure data acquired using a piezoelectric sensor. In terms of the mean pressure and temperature, only minor differences were found. The specific fuel consumption was improved under low load operation for the dual fuel mode by 1.8%, while a small increase of 1.2% was found near full load. Differences were found in the combustion process from 25 to 75% load with considerably faster premixed and diffusion combustion for the dual fuel mode leading to a 6−8% decrease in combustion duration. Despite the combustion process differences, the performance under dual fuel operation was overall close to that of conventional diesel with an acceptable 1.5% efficiency reduction on average. This confirms that modern dual fuel marine engines can achieve the performance standards of conventional ones while benefiting from low-carbon fuel use to reduce CO2 emissions.