The paper focuses on the technical challenges of producing high-quality welds in modern extreme climate conditions structures, as welds are typically the weakest part of welded structures. Welding is particularly difficult with high-strength and ultra-high-strength steels (HSS-UHSS), which are used in structures to reduce weight. The microstructural compositions and mechanical properties of dissimilar high-strength and ultra-high-strength steels were investigated in this study, which was performed with three different heat inputs (0.8, 1.2, and 1.8 kJ/mm). There was a 2.3Cr, 0.4Si, and 2.8Mn increase on the FGHAZ microstructure of the S960QC side, confirming the temperature increase in that zone. Microhardness results show softening (160 HV5) in the E500 side’s fine grain heat-affected zone (FGHAZ). Bending test results show that when the maximum force applied was 4000N, the fracture angle was close to 149°, and that the fracture zone was oriented exclusively in the FGHAZ, which had the higher softening zone. Tensile results show the fracture zone, which was oriented in the E500 side’s FGHAZ. It was suggested that a heat input of 1.2 kJ/mm be applied to the weld dissimilar joint of TMCP E500-S960QC, which will be beneficial for extreme climate conditions.