The district of Cefn Mably makes up part of St Mellons, a small suburb on the east side of Cardiff.
The district was formed by the Local Government Act 1972, from the county borough of Cardiff, the parishes of Lisvane, Llanedeyrn, Radyr, St. Fagans and Tongwynlais from the Cardiff rural district in the administrative county of Glamorgan and the parish of St Mellons from the Magor and St Mellons Rural District in the administrative county of Monmouthshire.
The district was formed by the Local Government Act 1972, from the county borough of Newport, the Caerleon Urban District and Magor and St Mellons Rural District (except the parishes of Henllys and St Mellons) from the administrative county of Monmouthshire.
A David Kemeys, the son of Ievan Kemeys of Began (near St Mellons) married Cecil, a daughter of Llewelyn ab Evan ab Llewelyn ap Cynfig of Cefn Mably in or around 1450.