Monmouth was first enfranchised (as the borough of Monmouth or Monmouth Town) during the reign of Henry VIII, at the same time as the counties and boroughs of Wales, and although it was legally regarded as being in England its electoral arrangements from the outset resembled those of the Welsh boroughs rather than those in the rest of England - it elected only a single member, and the borough consisted not only of the town after which it was named but also of a number of other "contributory boroughs" in the same county, which were required to contribute to the members' expenses and which had the right to send voters to take part in the election at the county town. In the case of Monmouth, there were initially six or perhaps seven contributory boroughs: Caerleon, Newport, Trellech, Usk, Chepstow, Abergavenny and possibly Grosmont; but by the late 17th century all of the electors were freemen of Monmouth, Usk and Newport.
The franchise was settled by a judgment in a disputed election in 1680, when Monmouth attempted to return a member to parliament without the involvement of the other boroughs, and the right to vote was declared to rest in the resident freemen of Monmouth, Newport and Usk. The number of electors seems once to have been substantial but to have fallen away sharply during the 18th century - from 2,000 in 1715 to about 800 in the 1754-1790 period; by the time of the Great Reform Act in 1832, there were only 280 qualified voters - 123 in Newport, 83 in Monmouth and 74 in Usk. In Tudor times the constituency was under the influence of the Duchy of Lancaster and around the start of the 18th century it was a pocket borough of the Morgan family of Tredegar, who were influential in the Newport area; but soon afterwards the Dukes of Beaufort gained control. After the Duke's candidate had won the election of 1715 decisively, this patronage was so clear that there were no further contests until 1820, the Beaufort candidates (many of them members of the family) all being returned unopposed for a hundred years.
At the time of the Great Reform Act the constituency had a population of just over 11,000 (of which Monmouth and Newport each contributed around 5,000 and Usk just over 1,000). This was a relatively large population for a borough constituency at the time - indeed, boroughs which had 2 MPs were generally allowed to keep them both under the Reform Act provided they had a population of 4,000. Nevertheless, all three of the component boroughs were enlarged slightly by including parts of the town outside the old borough, so bringing the combined population of the revised constituency to an estimated 13,101 and its electorate (under the reformed franchise) to 899. From this point onwards, the constituency was generally referred to as the Monmouth Boroughs.
From 1832 until the end of the 19th century the constituency was generally a marginal one, finely balanced between the Conservatives and Whigs or Liberals when it was contested (although Crawshay Bailey was returned unopposed four times after he was first elected). The constituency moved steadily towards the Liberals, however, as Newport grew in size; by the turn of the century 90% of the electorate was there, and it was a much more working class and industrial town than Monmouth or Usk. The Conservatives won in their landslide year of 1900 and held the seat in the by-election when the original election was declared void for various irregularities, but were probably helped by the association of the Liberal candidate with the campaign to extend the Welsh Sunday Closing Act to Monmouthshire. Otherwise, it was an increasingly safe Liberal seat, and at the time of the 1911 census had a population of 77,902.
The constituency was abolished by the Representation of the People Act 1918, with Newport becoming a parliamentary borough in its own right while Monmouth and Usk were included in the Monmouth county constituency.