It comprised the parishes of Bettws, Bishton, Coedkernew, Duffryn, Goldcliff, Graig, Henllys, Kemeys Inferior, Langstone, Llandevenny, Llanmartin, Llanwern, Llanvaches, Machen Lower, Magor, Marshfield, Michaelstone-y-Fedw, Nash, Penhow, Peterstone Wentloog, Rogerstone, Rumney, St Brides Wentloog, St Mellons, Redwick, Whitson and Wilcrick.
In 1938 the parish of Rumney was removed from the Rural District to become part of the county borough of Cardiff, in the neighbouring county of Glamorgan, although it remained part of Monmouthshire.
The Rural District Council comprised a number of councillors and a chairman.
Its responsibilities included sanitary services, sewerage, refuse collection, maintaining local roads, cemeteries and parks, licensing of public entertainments, water supply and housing.
Magor and St Mellons Rural District was abolished in 1974, with Henllys becoming part of the Torfaen district (borough) and St Mellons becoming part of the City of Cardiff.