Reactor pressure vessels (RPVs) are large forged components manufactured from Ni-Cr-Mo steel. RPVs typically exhibit a significant presence of martensite-austenite (M/A) islands within their microstructure after quenching. These islands deteriorate the mechanical properties of RPVs as they decompose into detrimental carbides during the subsequent tempering process. The current research investigates how the prior austenite grain size (PAGS) affects the morphology of M/A islands formed after quenching and its connection to the mechanical traits following tempering. A range of PAGS, spanning from 5 µm to 119 µm, is achieved by following the austenitization process at various temperatures and grain refinement thermal cycling methods. The outcomes demonstrate a direct relationship between increasing PAGS and a higher volume fraction of M/A islands in the quenched microstructure. Smaller PAGS predominantly leads to the formation of block-type M/A island morphology, whereas larger PAGS results in a mixture of block-type and film-type M/A islands. Upon tempering at 650˚C for 3 hours, scanning electron micrographs reveal the formation of globular precipitates of varying sizes due to the decomposition of block-type M/A islands. In contrast, rod-shaped precipitates are more common after the decomposition of slender-type M/A islands. Analyzing tempered microstructures shows a consistent increase in precipitate size as the PAGS of the specimen rises from 5 µm to 119 µm. These findings also highlight a significant correlation among PAGS, M/A island morphology, carbide formation and mechanical properties.