The present study focuses on dissimilar metal weld (DMW) of P91 ferritic-martensitic steel and Incoloy 800HT. The DMW was made nickel-based filler ERNiCrCoMo-1 filler using Tungsten Inert Gas (TIG) welding. Exhaustive microstructure analysis was done for the DMW. A stable austenitic microstructure was observed in the DMW solidified weld fusion zone (WFZ). Based on the peak welding temperature exposure in the different regions, the corresponding microstructure was observed to investigate the grain size transformations, phase transformations, diffusion, and segregation of constituent solute elements. Advance microstructure analysis was performed using Electron Probe Micro Analyzer (EPMA). Further, the effect of the observed microstructure feature on the mechanical property of the DMW was studied. The developed residual stresses were relaxed by post-weld heat treatment (PWHT). The tensile strength of the DMW was recorded less than the tensile strength of base metals. Also, impact toughness in as-welded (AW) WFZ was 117 J and reduced to 102 J after PWHT. Microhardness analysis showed maximum value in P91 heat-affected zone (HAZ) due to the dissolution of precipitates. It was reduced through tempering. The DMW of P91 and Incoloy 800HT was successfully produced without any major defect and with sufficient mechanical strength to put it in consideration for use in advanced ultra-supercritical (AUSC-Generation IV) boiler pipes and tubes as per the ASTM standards. © 2022