English. The ONC gives the arrival of Wilhelmina and its pet-name Minnie in the UK as a part of the wave of Germanic names after Queen Victoria married Prince Albert of Saxe-Coberg-Gotha. With the advent of the first world war, Germanic names fell out of favour. Minnie was also used as a pet-name for the Scottish equivalent, Williamina.
Minnie as a nick-name for Mary is widespread in some communities and often reduced to Min.
A correspondent [LPC] has a family connection with a Maria, born in London, UK, in 1853 who was known as Minnie. Whether this use was common in the UK is not known and the official documents only used her given name.
Minnie has occasionally been used as a given name.