Shipping is a significant contributor to global greenhouse gas (GHG) and air pollutant emissions. This study uses a life cycle assessment to compare emissions from domestic and imported liquefied natural gas (LNG), and heavy-fuel oil (HFO) for marine shipping. The findings show that only high-pressure dual-fuel (HPDF) engines robustly reduce well-to-wake GHG emissions by 10% compared with their HFO-fuelled counterparts. This engine technology is only available for large low-speed engines used in ocean-going vessels (OGVs). For smaller vessels, such as ferries, the current deployment of medium speed low-pressure dual-fuel (MS-LPDF) and lean burn spark ignition (LBSI) gas engines cannot reliably reduce GHG emissions. This is primarily due to the high levels of methane slip from these engines. For air pollution reduction, gas engines are found to be an effective means of reducing nitrogen oxides, sulphur oxides and, particulate matter without any additional engine aftertreatment. The HPDF engines, however, need aftertreatment or exhaust gas recirculation to meet the International Maritime Organization Tier III regulations. Sulphur controls, such as the 2020 act, move to limit sulphur to 0.5% globally. However, this will increase the cost of the HFO used by most OGVs, enhancing the economic case for natural gas fuel.