The area of present-day Carlow has been inhabited for thousands of years, and the county has perhaps the highest concentration of megalithic monuments per square mile in Ireland.

The historic clan territories of the county included Uí Drona (O'Ryan), Fothairt Feadh (O'Nolan), Uí Ceinnselaig (Kinsella), Dál Coirpri Cliach (Kerwick), Uí Bairrche Magh dá chonn (Kearney), Uí Felmeda Tuaidh (O'Garvey) and Uí Bairrch Maige hAilbe (O'Gorman).

With the exception of a short-lived Norse–Gael settlement near St. Mullin's in the 9th century, the area remained under the control of the Kingdom of Leinster until the 13th century.

Following the Norman conquest, the "Borough of Carlow" was founded in July 1210, and formed part of the Norman palatine county of Leinster. This was later divided, and the independent Liberty of Carlow was established in 1247. At that time the county was over three times larger than it is today,

The alliance between the Kingdom of Leinster and the Anglo-Normans remained the status quo for decades, as it kept the peace and made both sides immensely wealthy. Cognizant of the political landscape, the Anglo-Normans began to marry into Gaelic families and adapt to native customs, forging alliances with Irish kingdoms to gain the upper hand over their fellow Anglo-Norman rivals. In a bid to halt the decline of English authority in the region, the Crown made Carlow the capital of the Lordship of Ireland from 1361 until 1374.

The modern county boundary was shaped by the Gaelic Resurgence in the 14th to 16th centuries. During this period, Carlow was part of the patrimony of the Anglo-Norman Butler dynasty; however Art MacMurrough-Kavanagh, the ascendant King of Leinster, controlled more than half of the liberty.

In 1606 the eastern and northern parts of Carlow were made part of the new county Wicklow.

Following the Irish Confederate Wars in the 1650s, the great majority of Gaelic Irish and Anglo-Norman landowners were dispossessed, and their lands were granted to English soldiers who took part in the Cromwellian conquest.

Carlow, along with neighboring Wexford, saw some of the fiercest fighting of the 1798 Rebellion. The rebellion in Carlow is particularly infamous for the sectarian excesses committed within the county by members of the Orange Order.

During the War of Independence, the Carlow Brigade of the Irish Republican Army (IRA) had 6 battalions which operated in the shadow of the Curragh Camp – the British Army's headquarters.

The Carlow Republican District Court, established in February 1922 at the Carlow Courthouse, was the first post-independence court held by the government of the Irish Free State.