Pathogenic, bi-allelic variants in RMND1 typically manifest as an early-onset multisystemic mitochondrial disease. Although in vitro evidence suggests a role for RMND1 in mitochondrial protein synthesis, its function in vivo has remained poorly defined. To answer this question, we have generated and characterized the first in vivo model of RMND1-disease. Rmnd1 homozygous mutant mice (Rmnd1N238S) exhibit a severe multisystem phenotype marked by widespread defects in oxidative phosphorylation (OXPHOS) assembly and activity defects. Quantitative proteomics analysis of Rmnd1N238S heart indicates profound metabolic remodeling, with concomitant significant alterations in the profile of mitochondrial ribosome proteins. These changes point to a late-stage assembly defect of the mitochondrial ribosome large subunit, a defect recapitulated by both RMND1 mutant and RMND1-depleted human cells. Together, our findings establish Rmnd1N238S mice as a robust in vivo model of RMND1-related disease and uncovers the key molecular defects that link RMND1 function to mitochondrial ribosome assembly and translation in vivo.