The Diocese of Monmouth was created in 1921, when the Church in Wales was 'disestablished' from the Church of England. The diocese was created from the eastern part of the existing Diocese of Llandaff and covers more or less the territory of the traditional county of Monmouthshire. The first Bishop was elected in 1921.

In its own words, the diocese "covers the south east corner of Wales, from Monmouth south to Chepstow, westwards along the 'M4 corridor' to Newport and the outskirts of Cardiff, northwards into the south eastern valleys and east into the rural areas around Usk, Raglan, Abergavenny and the Herefordshire border". The territory of the Diocese now covers the City of Newport, all of the counties and county boroughs of Blaenau Gwent, Monmouthshire and Torfaen, parts of the City of Cardiff and the county borough of Caerphilly, and even a (very) small part of Herefordshire in England.

The Bishops of Monmouth since 1921 have been:

• 1921 Charles Alfred Howell Green
• 1928 Gilbert Cunningham Joyce
• 1940 Alfred Edwin Monahan
• 1945 Alfred Edwin Morris (also Archbishop of Wales 1957-1967)
• 1968 Eryl Stephen Thomas
• 1972 Derrick Greenslade Childs (Archbishop of Wales 1983-1986)
• 1986 Royston Clifford Wright
• 1991 Rowan Douglas Williams (Archbishop of Wales 1999-2002)
• 2003 Edward William Murray (Dominic) Walker

The current Bishop, Rt Rev Dominic Walker OGS, was previously Area Bishop of Reading in the Church of England. His predecessor, Rt Rev Rowan Williams, was appointed Archbishop of Canterbury in 2002 - the first Welsh bishop to hold that post since the English Reformation in the sixteenth century.