It is uncertain when they first arrived on the islands, but it is possible that they unwittingly were transported there during the Norse period. Isolated on the islands, the St Kilda house mouse diverged from relatives. It became larger than the mainland varieties, although it had a number of traits in common with a subspecies found on Mykines in the Faroe Islands, Mus musculus mykinessiensis.
When the last St Kildans were evacuated in 1930, the endemic house mouse became extinct very quickly, as it was associated strictly with human settlement. Some specimens exist in museums. The St Kilda field mouse (Apodemus sylvaticus hirtensis) is still present.
Musser, G. G. and M. D. Carleton. 2005. "Superfamily Muroidea". Pp. 894-1531 in Mammal Species of the World a Taxonomic and Geographic Reference. D. E. Wilson and D. M. Reeder eds. Johns Hopkins University Press, Baltimore.