In the 1861 census, William Mather, living in the West Derby area near Liverpool, was recorded as at home with parents, aged 23, single, occupation "Lieutenant, West York Malitia" [sic].
A visitor to the family was also 23, a cornet in 14th Light Dragoons.
In later censuses William has no similar occupation. I should be interested to know where to find anything about the West York Militia, as the family was firmly rooted in Lancashire.
Ainslie