Mating behaviour is an important component of species’ life histories. Knowledge of natural patterns of mating can lead also to more effective management strategies for populations of conservation concern. Despite a high conservation profile many aspects of the biology of the common dormouse (Muscardinus avellanarius) remain unknown, potentially limiting present conservation efforts. We determine the mating behaviour of M. avellanarius at two woodland sites in the UK: (1) Bontuchel (a natural population in Wales) and (2) Wych (a population in England that was established by reintroducing captive-bred animals) by genotyping mothers and litters at a panel of 10 microsatellite loci. Adult female body weight positively correlates with litter size and no apparent reproductive skew was evident. We found that multiple mating by female dormice is prevalent at both sites, with litters containing three or more offspring sired by multiple fathers; moreover, multiple mating is adopted by released animals even after a period of captive breeding where females are mated singly or as a breeding pair. We also present evidence for low proportion of fathers identified in our samples that probably related to unsampled individuals and/or larger than anticipated population sizes. This first report of mating behaviour in M. avellanarius highlights the role of genetic studies to uncover species’ reproductive behaviours and include these data for conservation management.