According to the Act the shire consisted of all Honours, Lordships, Castles, Manors, Lands, Tenements and Hereditaments, lying or being within the Compass or Precinct of the following Lordships, Townships, Parishes, Commotes and Cantrefs... in the Country of Wales: Monmouth • Chepstow • Matherne (Mathern) • Llanvihangel (Llanfihangel Rogiet) • Magour (Magor) • Goldcliffe (Goldcliff) • Newport • Wentlooge • Llanwerne (Llanwern) • Caerlion (Caerleon) • Usk • Treleck (Trellech) • Tintern • Skenfrith • Grosmont • Witecastle (White Castle) • Raglan • Calicote (Caldicot) • Biston (Bishton) • Abergavenny • Penrose (Penrhos) • Grenefield (Maesglas) • Maghen (Machen) • Hochuyslade (possessions of Llanthony Priory)[3] The Act also designated Monmouth as the "Head and Shire town of the said county or shire of Monmouth", and ordered that the Sheriff's county or shire court be held alternately in Monmouth and Newport.

The Diocese covered all of the County of Monmouthshire but also included the Parish of Cyncoed in north-east Cardiff, a situation which continues in 2023.

According to historian John Davies, this arrangement was the origin of the belief that the county had been annexed by England rather than remaining part of Wales.

[15] The Modern Universal British Traveller of 1779 stated: "Monmouthshire was formerly a part of Wales, and continued so till the reign of Charles II, when it was reckoned an English county (as it has been ever since) because the judges then began to keep the assizes here in the Oxford circuit.

According to Chris Williams of Cardiff University:[21]"...The problem with Monmouthshire is that it was located not on, strictly speaking, a national frontier, but within a single political formation.

This had been the case since the Acts of Union of 1536 and 1543 which had extended the English system of justice, government and parliamentary representation to the principality of Wales and the March.

Once specific policies (be they considered corrective or emancipating) began to be designed for application to Wales, then the question of Monmouthshire's placement became important.

Some of these, and others with "social aspirations", considered it essential to emphasise their "Englishness", and there were attempts to refine the notion that anomalies in the 16th-century legislation had made the county non-Welsh.

The distinction implied in the description "Wales and Monmouthshire" was nurtured by elements of the establishment, and became increasingly accepted on the English side of the border and in central government, until the local Welsh residents more fully asserted themselves in the 20th century.

The question of Monmouthshire's status continued to be a matter of discussion, especially as Welsh nationalism and devolution climbed the political agenda in the 20th century.

In 1921 the Earl of Plymouth objected strongly to the inclusion of the county in legislation forcing the closing of public houses in Wales on Sundays.

[24] During a debate on the Administration of Justice (Miscellaneous Provisions) Bill in 1938, Lord Raglan objected to the stipulation that the chairman of the Monmouthshire quarter sessions should be a Welsh-speaker.

[25] In 1948, Monmouthshire County Council was granted a coat of arms with the motto Utrique fidelis ("Faithful to both"), to reflect the dual loyalties of the area.

[24] Increasingly in legislation, "the state was prepared to treat Monmouthshire as if it were part of Wales, even if it was not able to concede that legally it was Welsh.

The Member of Parliament (MP) for Abertillery, Llywelyn Williams, campaigned to have Monmouthshire unambiguously placed in Wales.

[31] In reaction, a "Make Monmouthshire Welsh" campaign was launched in August 1961 to gain parliamentary recognition that the county was in Wales.

[32] In November 1961 it was announced that a branch of the Royal Society of St George was to be formed, emphasising the county's English identity.

[33] Another group, the "Monmouthshire is Welsh" society, was formed and was supported by local MPs Michael Foot and Leo Abse.

[24] Two years later Monmouth Borough Council made representations to the Minister of Housing and Local Government seeking the transfer of the town to Herefordshire in the event that the reforms were carried out.

The Interpretation Act 1978 hence provides that in legislation passed between 1967 and 1974, "a reference to England includes Berwick upon Tweed and Monmouthshire".

Replying for the government, Baroness Phillips stated that "The purpose of the change is to remove the anomaly arising from the present need to refer to Monmouthshire separately from Wales in Statutes.

"[36] In April of the following year Plaid Cymru MP Gwynfor Evans asked Thomas "when he proposes to implement his undertaking that the phrase 'Wales and Monmouthshire' " will be dropped."

[37] The issue was finally clarified in law by the Local Government Act 1972, which provided that "in every act passed on or after 1 April 1974, and in every instrument made on or after that date under any enactment (whether before, on or after that date) "Wales", subject to any alterations of boundaries..." included "the administrative county of Monmouthshire and the county borough of Newport".

Raymond Gower said that the statement was "quite consistent with the fact that towns with names such as Abergavenny and Llantarnam could not be anything other than Welsh"; it was opposed by Gerald Kaufman, who stated: "Am I to take it that an act of annexation of this magnitude is to be carried through a sparsely attended House of Commons on the nod at five minutes to midnight on the same basis as Europe has annexed England?

The preserved county of Gwent, which still exists for some ceremonial purposes, is similar in extent to historic Monmouthshire with the addition of the west bank of the Rhymney Valley.