Mitochondrial ribosomes have a central importance as they synthesize the core subunits of the oxidative phosphorylation system, which produces the majority of cellular energy. Defects during mitochondrial ribosome biogenesis leads to OXPHOS deficiency and subsequently to severe early-onset diseases, yet how this complex machinery assembles is poorly understood. Here, we reconstructed the first comprehensive assembly map for the human mitochondrial ribosome – from very early to late assembly steps. In contrast to bacterial ribosomes, mitochondrial ribosomes assemble via protein-only modules, which are produced in excess and which present primed states for subunit biogenesis. The formation of these protein-only complexes is independent of the ribosomal RNA and our data suggest that rRNA synthesis is the rate-limiting step in the progression of ribosome assembly in human mitochondria. This also rationalizes how mitochondria coordinate the formation of these large protein-rich complexes from dual genetic origin.