In form at least the surname is Scottish, deriving from the place of the same name in Annandale in Dumfrieshire, which was originally "John's town". The original John was a Norman landowner in the area in the twelfth century, and instead of taking on the straightforward patronymic "Johnson", his descendants adopted the placename as their surname, becoming Johnston(e)s. This family, the source of virtually all Scottish bearers of the name, became one of the strongest and most unruly of the Border clans, and their long feud with another clan, the Maxwells, was notorious for its ferocity. When the clans were eventually "pacified" and scattered by James II, many Johnstons fled to Ulster where, like large numbers from the other clans - Elliots, Armstrongs, Nixons and others - they settled mainly in Co. Fermanagh, where the surname is today the second most numerous in the county. As well as these Johnstons, however, many others whose name was originally Johnson adopted the Scottish name. Such adoptions occurred predominantly in Ulster, and affected those of Scottish and of native Irish origin, with the MacIans of Caithness translating their surname as Johnson, and then altering it to Johnston in many cases, and the MacShanes of the Armagh/Tyrone district, a branch of the O'Neills, doing likewise.

Johnston households by county in the Primary Valuation property survey of 1848-64.
Click on a county name for a parish breakdown(paying).
Antrim 363
Armagh 149
Belfast city 210
Carlow 2
Cavan 135
Clare 4
Cork 28
Cork city 18
Derry 163
Donegal 134
Down 401
Dublin 52
Dublin city 69
Fermanagh 415
Galway 25
Kerry 13
Kildare 13
Kilkenny 11
Laois 27
Leitrim 61
Limerick 5
Limerick city 1
Longford 16
Louth 70
Mayo 30
Meath 30
Monaghan 129
Offaly 43
Roscommon 28
Sligo 56
Tipperary 20
Tyrone 285
Waterford 7
Westmeath 20
Wexford 27
Wicklow 29