Ammonia production is responsible for around 35% of the CO2 emissions associated with the global chemical sector. In this study, a bottom-up model with high technical and economic resolution is implemented to assess potential least-cost pathways to achieve a net-zero U.S. ammonia industry by 2050. Results indicate that natural gas reformers equipped with carbon capture technologies may play a key role in decarbonizing U.S. ammonia production in the near term, while water electrolysis will be required in the long term to achieve net-zero emissions. Technical improvements and cost reductions in electrolyzers and lower-cost renewable electricity will be needed to accelerate the adoption of water electrolysis. Furthermore, two key policy interventions were investigated (carbon pricing and production tax credits) and were found to hold significant potential for accelerating low-carbon ammonia production technologies and for further reducing the cumulative carbon emissions. The methodology and analyses presented in this study provide important insights into available transition pathways to decarbonized U.S. ammonia production, providing fundamental insights for policy makers on required actions and investments to achieve the net-zero emissions targets.