The application of structural metals in extreme environments necessitates materials with superior mechanical properties. Mo-doped FeCoCrNi high-entropy alloys (HEAs) have emerged as potential candidates for use in such demanding environments. This study investigates the hightemperature performance of FeCoCrNiMox HEAs with varying Mo contents (x = 0, 0.1, 0.3, and 0.5) prepared by laser powder bed fusion additive manufacturing. The mechanical properties were evaluated at room and 600 degrees C temperatures, and the microstructures were characterized using scanning electron microscopy, electron backscatter diffraction, energy dispersive X-ray spectroscopy, and transmission electron microscopy. The intrinsic dislocation cell patterning, solid-solution strengthening, nanoprecipitation, and twinning effects collectively modulated the plastic deformation behavior of the samples. The high-temperature mechanical performance was comprehensively analyzed in conjunction with ab initio calculations and molecular dynamics simulations to reveal the origin of the experimentally observed strength-ductility synergy of FeCoCrNiMo0.3. This study has significant implications for FeCoCrNiMox HEAs and extends our understanding of the structural origins of the exceptional mechanical properties of additively manufactured HEAs.