Scimitar engine is a hypersonic hybrid engine designed to propel the LAPCAT A2 aircraft. In this study, a novel exergy and NOx emission-based ecological performance analysis of the engine is performed. For this purpose, first, a component-based exergy analysis for the cruise phase of the Scimitar engine in air-turborocket mode is performed and the exergy destruction rates of engine components are determined at Mach 5 by the thermodynamic model developed. Then, a novel objective function, the coefficient of emission-based ecological performance (CEEP) is defined as “the propulsive power produced per unit environmentally harmful exhaust gas emission rate,” which can be utilized to assess the ecological impact of any jet engine. Finally, the impacts of cruise speed, altitude, and air and fuel mass flow rates on the exergetic and NOx emission-based ecological performance of the engine are investigated by the aid of the newly defined CEEP, together with the exergy efficiency and the coefficient of ecological performance. It is found that the combustion chamber is responsible for 57.36% of the overall exergy destruction rate of 123.80 MW at the cruise conditions, and CEEP relatively increases by 13.8% when the hydrogen fuel consumption rate is increased from 3.96 kg/s to 4.17 kg/s. Increasing the cruise speed from Ma = 4.88 to Ma = 5.2 and decreasing the altitude from 25 km to 23 km cause a relative degradation of 12.75% in CEEP.