The gas tungsten arc welding (GTAW) process is regarded as the successful modus operandi in making the sound quality dissimilar welds joint for advanced ultra-supercritical (AUSC) boilers. However, heat input and filler metal application are critical to the structural strength of the welded joint. This study aimed to explore the microstructure, and mechanical properties of a ferritic/martensitic grade P91 steel and Incoloy 800HT welded joint created utilizing a multi-pass GTAW process with the AWSER90S–B9 (9CrMoV–N) filler. With a groove angle and root gap of 75° and 1.5 mm, respectively, the V groove butt design was adopted. The welded joint was characterized in the as-welded (AW) state and the post-weld heat treated (PWHT) state. The characterization included microstructural investigation of the weldment as well as mechanical testing. The microstructural understanding reasons for the mechanical behavior of the (DMW). The measurements of mechanical properties, such as the behavior of microhardness in different regions of the weld, tensile properties, and impact toughness, all were done at room temperature and per the standards. Poor impact toughness and high weld metal hardness were the results of the as-welded (AW) weld metal's untempered martensitic lath microstructure. Along the weldments, non-uniform microstructure growth of various grain sizes and precipitate densities was seen, leading to nonlinearity in mechanical characteristics. The capability of the weld metal to resist the impact load, i.e., its impact toughness, in correspondence to an increase in a slight increase in tensile strength after PWHT, was found to fall from 96 J in AW state to 80 J PWHT state. The PWHT proved to be crucial for AWSER90S–B9 (9CrMoV–N) filler dissimilar weld as it enhanced the mechanical strength. PWHT significantly decreased the surges in the mechanical parameters along the weldments, such as hardness but at the cost of impact toughness. © 2022 Elsevier Ltd