It is in the province of Ulster and is part of Border strategic planning area of the Northern and Western Region.

[2] The county has existed since 1585 when the Mac Mathghamhna rulers of Airgíalla agreed to join the Kingdom of Ireland.

In 1585, the English Lord Deputy of Ireland, Sir John Perrot, visited the area and met the Irish chieftains.

They requested that Ulster be divided into counties and land in the kingdom of Airgíalla be apportioned to the local chiefs.

The county was subdivided into five baronies: Farney, Cremorne, Dartrey, and Monaghan controlled by MacMahon and Truagh by McKenna.

[12]: xiii  The partition of Ireland in 1922 turned the boundary with County Armagh into an international frontier, after which trains were routinely delayed by customs inspections.

He was a prolific exhibitor at the Royal Hibernian Academy throughout his lifetime and is represented by works in the collection of the National Gallery of Ireland and the Ulster Museum.

A Catholic convert, Irish nationalist and first cousin of Sir Winston Churchill, Prime Minister of the United Kingdom, Leslie became an important literary figure in the early 1900s.

He was a close friend of many politicians and writers of the day including the American novelist F. Scott Fitzgerald (1896–1940), who dedicated his second novel, The Beautiful and Damned, to Leslie.

The best of the county's architecture developed in the Georgian and Victorian periods and ranges from the dignified public spaces of Church Square and The Diamond in Monaghan Town to the great country houses of Lough Fea, Carrickmacross; Hilton Park, Clones and Castle Leslie, Glaslough.

Agriculture is a significant part of the County Monaghan economy, employing about 12% of the population in 2011 (compared with 5% nationally).