A detailed investigation has been carried out of the matching composition filler metal for welding MarBN steel to explore the influence of welding procedure and post-weld heat treatment (PWHT) on the microstructure and properties of the weld metal and a MarBN steel weld joint. The filler metal was in the form of a shielded metal arc welding (SMAW) electrode. With different interpass temperatures and PWHT procedures applied, the microstructure and mechanical properties, particularly toughness, of the all-weld metal deposits and welded MarBN alloy joints were examined. The differences in microstructure (grain structure, precipitates, etc.) were evaluated to provide a better understanding of how the PWHTs influence the starting conditions of the matching weld metal and welded MarBN steel joints before service. The current work found that for a 30-mm-thick all-weld metal coupon, reasonable impact toughness was achieved after PWHT of 735 °C × 8 h; longer heat treatment durations (16 h and 36 h) did not further increase the toughness. For thicker cast blocks, a PWHT of 735 °C × 16 h was needed for weld toughness comparable to the base metal, hence was considered more appropriate for actual engineering applications. Evenly distributed Laves phase precipitation was observed after 735 °C × 8 h; the precipitates gradually increased in size and area fraction as the duration increased, and appeared to be stable when approaching 36 h. Differences in the precipitation were observed between different weld regions. The weld cap region had a lower density of Laves precipitates compared to the lower weld area. From the current investigation, aspects for further investigation are also identified.