RNase MRP and RNase P are evolutionarily related complexes that facilitate rRNA and tRNA biogenesis, respectively. The two enzymes share nearly all protein subunits and have evolutionarily related catalytic RNAs. Notably, RNase P includes a unique subunit, RPP21, whereas no RNase MRP-specific proteins have been found in humans, limiting molecular analyses of RNase MRP function. Here, we identify the RNase MRP-specific proteins, C18orf21 (RMP24) and NEPRO (RMP64). C18orf21/RMP24 and RPP21 display significant structural homology, but we identify specific regions that drive interactions with their respective complexes. By targeting these RNase MRP-specific subunits, our functional analysis reveals that RNase MRP is essential for rRNA processing and preferentially required for 40S ribosome biogenesis. Finally, we determine that disease-associated mutations in RMP64 impair its association with RNase MRP subunits. Together, our findings elucidate the molecular determinants of RNase MRP function and underscore its critical role in ribosome biogenesis and disease.