Total greenhouse gas emissions (GHG) declined in 2020, around 6%, due to COVID-19 pandemic restrictions. Simultaneously, the renewable energy share on electricity production increased significantly; however, this trend is not expected to last. The COVID-19 pandemic has further highlighted the need for change paradigms to build more resilient and sustainable cities, including power and heat supply, transport, and logistics of basic supplies. We still rely heavily on fossil fuels, particularly in the transport sector. Therefore, finding sustainable alternatives to fossil fuels is imperative to decarbonize the world’s energy mix. In recent years, several researchers have studied the potential use of ammonia (NH3) as an energy vector, focused on the techno-economic advantages and challenges for full global deployment. The use of ammonia as fuel implies zero direct carbon emissions; however, to state the sustainability of the shift to ammonia as fuel in thermal power plants and other thermal engines, a study of the environmental profile is needed. Therefore, this paper aims to compare the environmental profile of grey versus green ammonia in a gas turbine power plant, using the life cycle assessment (LCA) methodology.