The county was officially formed in 1585 by Sir Henry Sidney, Lord Deputy of Ireland, but did not come into effect until the chaos of the Nine Years' War ended, in 1603.

Under the system of surrender and regrant each tuath was subsequently made into an English barony: Carbury, Tireragh, Tirerril, Leyny, Corran and Coolavin.

A causewayed enclosure discovered in 2003 at Maugheraboy is one of the earliest indications of Neolithic farming activity on the Cúil Irra Peninsula.

[4] The nearby megalithic cemetery of Carrowmore forms part of a huge complex of Stone Age remains connecting Carrowkeel in south Sligo to the Ox Mountains, to the Cuil Irra Peninsula, where the passage tomb named after the legendary Queen Maeve, Miosgán Médhbh, dominates the western skyline from the crest of Knocknarea Mountain.

A recent decoding[5] of the work of Marinus of Tyre and Ptolemy shows Sligo as the likely location of Nagnata, an important place of assembly in the Iron Age.

The patron of the Annals of the Four Masters was Fearghal Ó Gadhra of Coolavin in south County Sligo.

The design on the black shield, which shows an open book on which there is a Celtic Cross and a red rose, represents collectively the literary and cultural history of Sligo.

These refer to such early works as the Books of Ballymote and Lecan, while the rose was a significant theme in the poetry of W. B. Yeats.

The rich array of megalithic monuments in the area has been an inspiration to artists and writers such as Sir Samuel Ferguson.

The poet and Nobel laureate W. B. Yeats (1865–1939) spent much of his childhood in northern Sligo and the county's landscapes (particularly the Isle of Innisfree, in Lough Gill) were the inspiration for much of his poetry.

The south of the county is particularly noted with such musical luminaries as James Morrison, Michael Coleman, Paddy Killoran, Fred Finn, Peter Horan, Joe O'Dowd, Jim Donoghue, Martin Wynne, Oisín Mac Diarmada (of Téada), tin-whistle player Carmel Gunning and the band Dervish.