Gas Tungsten Arc Welded dissimilar joint of modified 9Cr–1Mo (P91) and SS304H steel plate is prepared using Ni-based ERNiCrCoMo-1 super alloy filler and are post-weld heat treated (PWHT) at 760 °C for 2 h. In particular, joints are investigated for metallurgical and mechanical properties in different heat-affected zones. Furthermore, an attempt has been made to understand the following: (i) Is there any effect of mixing of Ni-based superalloy filler material on the joint? (ii) How does the PWHT process change the metallurgical and mechanical properties of joints and different sections? (iii) Does the PWHT process beneficial as compared to as-welded (AW) joint? Experimental results indicate that improper mixing of filler materials led to the formation of an unmixed zone near the weld metal and HAZ interface. The hardness, residual stress, and impact energy results confirmed the variation in mechanical properties along the weldments. The tensile strength varied from 539 MPa in as-joint to 516 MPa in PWHT, and impact energy varies from 165 J in as-joint to 40 J in PWHT condition. The maximum hardness of 375 H V is observed in P91 CGHAZ. The maximum longitudinal residual stress is estimated as 200 MPa at the top surface of the welded plate, i.e., about 2 mm depth, while the maximum magnitude of the transverse residual stresses is observed as 109 MPa at the same depth of 2 mm. In P91 HAZ, the longitudinal and transverse residual stress magnitudes are found to be in a range of 0 to ˗70 MPa. The SS304H HAZ shows the maximum value of longitudinal and transverse residual stress level at a depth of 3 mm, and it was 144 MPa and 92 MPa, respectively. The PWHT showed a drastic reduction in residual stress value in a different zone of the weldments. © 2021 Elsevier Ltd