The Eastern Arc Mountains in Tanzania represent a hotspot for biological diversity of global importance. The level of endemism is high, and Eastern Arc biodiversity has been studied extensively in vertebrates and invertebrates, including millipedes. However, millipede evolution is vastly understudied at the molecular level. Therefore, we used next-generation ‘shotgun’ sequencing to obtain mitochondrial genome sequences of 26 museum specimens, representing six genera and 12 millipede species found across the Eastern Arc Mountains. Bayesian and maximum likelihood methods yielded consistent topologies with high node support, confirming a high level of congruence between molecular and morphological analyses. The only exception was a Tropostreptus sigmatospinus individual from Zanzibar, which was placed outside an otherwise monophyletic group consisting of mainland individuals of the same assumed species. For two species with a distribution across several mountain blocks (Tropostreptus sigmatospinus and Tropostreptus hamatus), each mountain population represents a distinct monophyletic lineage. In contrast, we also observe that distinct species exist sympatrically in the same montane forests, indicative of older speciation events that are not defined by current forest distribution. Our results are important for understanding speciation mechanisms in montane rain forests and highlight that ethanol-preserved invertebrates exhibit a tremendous potential for genomic analyses.