The global drive to limit the effects of climate change affords a strong incentive to reduce CO2 emissions. H2 is one of the cleanest energy sources, because its combustion does not produce CO2. It is well known that NH3 stores as well as carries H2 and does not produce CO2 on combustion. NH3 has been investigated for its use as an alternative fuel and for use in internal combustion engines. Investigations of NH3 and N2O emissions from NH3-assisted diesel engines operated using NH3–diesel dual fuel have been scarce. NH3 and N2O cause air pollution and are toxic to humans; therefore, these pollutants should be reduced to acceptable levels. In addition, N2O is a greenhouse gas with high global warming potential. In this study, a combustion strategy was developed to reduce NH3 and N2O emissions from an NH3-assisted diesel engine. NH3 and diesel fuel worked as low- and high-reactivity fuels, respectively, in our strategy for reactivity-controlled compression ignition combustion. The present paper reports the insights obtained from an understanding of the chemical processes of diesel fuel ignition and NH3 decomposition. Experiments revealed the effects of advancing diesel pilot injection timing on emissions and combustion performance, and the manipulation of combustion phasing using a change in the amount of injected diesel fuel and NH3. Finally, the reduction in greenhouse gas emissions considering the global warming effects of N2O was estimated using an NH3-assisted diesel engine that applied the proposed combustion strategy.