Until the late sixteenth century, the area now called County Monaghan was simply part of the kingdom of Airgialla, controlled by the McMahon chieftains across most of the territory and by the McKennas in the northern barony of Truagh. In 1585 the English Lord Deputy of Ireland, Sir John Perrot, travelled north and met the Irish chieftains. They asked, perhaps unwisely, that Ulster be divided into counties and that land be apportioned to the local chiefs. A commission was established. County Monaghan was created, with five baronies: Farney, Cremorne, Dartrey, and Monaghan under the McMahons, and Truagh under the McKennas. After the defeat of the Earl of Tyrone in 1603 the county was, unlike the rest of Ulster, not subjected to plantation. The lands were left in native hands. When the McMahons joined the 1641 rebellion and lost, some Scottish and English settlement followed, but Monaghan never became a deeply planted county. It is part of why it sits awkwardly today between northern and southern Irish identities.

The county's geography is essentially one repeated idea: drumlins. These long egg-shaped hills, oriented roughly southeast to northwest, were carved by retreating Pleistocene ice and dropped across the county in their thousands. Between them lie small lakes: Lough Avaghon, Dromore Lough, Lough Egish, Emy Lough, Lough Fea, Muckno Lough, White Lough, and dozens more without names on most maps. Slieve Beagh, on the Tyrone and Fermanagh borders, is the highest mountain at 380 metres; Mullyash Mountain and Coolberrin Hill at 214 metres are the only other significant uplands. Rossmore Forest and Dartrey Forest, the two largest woodlands, are now managed by Coillte and are mostly conifer plantation. Native woodland is reduced to scattered pockets after centuries of intensive farming. The Finn Bridge near Scotshouse is the small border crossing into Fermanagh.

The poet Patrick Kavanagh was born in Inniskeen, in the southeast of the county, in 1904. He stayed on the family farm until his thirties before moving to Dublin. His best work is rooted in this county and its specific geography: the poem 'Stony Grey Soil' is a bitter address to the land itself ('O stony grey soil of Monaghan / The laugh from my love you thieved'); 'Shancoduff' describes the small black hills of his own farm. He is one of the most significant figures in twentieth-century Irish poetry, the man who made unfashionable rural Catholic Ireland into a viable subject for serious literature. He shares the county's literary roster with Patrick McCabe of Clones, the novelist who wrote The Butcher Boy and Breakfast on Pluto; Eugene McCabe of Clones, who wrote Death and Nightingales; and the playwright and director Sir Tyrone Guthrie, who settled at Annaghmakerrig House in his later years and left it to the Irish state as the Tyrone Guthrie Centre artist residency.

The best of Monaghan's architecture is Georgian and Victorian. The dignified public spaces of Church Square and the Diamond in Monaghan Town, the county seat, are matched by the country houses scattered through the demesnes: Lough Fea near Carrickmacross, Hilton Park near Clones, and most famously Castle Leslie at Glaslough in the northeast corner. Castle Leslie was the home of the Leslie baronets, including Sir Shane Leslie, third baronet, the writer and Catholic convert who was a first cousin of Sir Winston Churchill. Leslie was a literary figure of some weight in the early 1900s and a close friend of F. Scott Fitzgerald, who dedicated his second novel, The Beautiful and Damned, to him. Significant ecclesiastical buildings include St Macartan's Catholic Cathedral in Monaghan Town, designed by James Joseph McCarthy and completed in the Gothic Revival style, and the rather sterner St Patrick's Church of Ireland Church on the other side of the square.

For a small county Monaghan has exported a startling variety of people. Joseph Finegan, born here in 1814, commanded the Confederate forces at the Battle of Olustee in 1864 during the American Civil War. Juan Mackenna, born in 1771, became a general of the Chilean War of Independence and is honoured in Chile as a co-liberator. Sir Charles Gavan Duffy, born in Monaghan town in 1816, served as Premier of the Australian colony of Victoria. John Robert Gregg, born in 1867, invented the shorthand system that now bears his name and that taught generations of stenographers worldwide. Sir William Whitla, the great Belfast physician, was born and raised in Monaghan town. The boxers Barry McGuigan and Kevin McBride and the rally driver Daniel McKenna are more recent exports. Eoin O'Duffy, born near Castleblayney in 1892, became Chief of Staff of the IRA, Commissioner of the Garda Siochana, leader of the Blueshirts, and commander of the Irish Brigade in the Spanish Civil War, an arc of political extremes few twentieth-century Europeans matched. Monaghan is a small place. The doors it has opened lead a long way out.