Tarn is one of the original 83 departments created during the French Revolution on 4 March 1790, through application of the Law of 22 December 1789. It was created from part of the former province of Languedoc, and comprised the dioceses of Albi and Castres

Tarn's three principal ranges lying to the south-east are: the Mountains of Lacaune, the Sidobre, and the Montagne Noire, belonging to the Cevennes. The stony and wind-blown slopes of the first named are used for pasture. The highest point of the range and of the department is the Pic de Montalet (about 4150 ft.); several other summits are not much short of this. The granite strewn plateaux of the Sidobre, from 1600 to 2000 ft high, separate the valley of the river Agout from that of its western tributary, the river Thoré. The Montagne Noire, on the southern border of the department, derives its name from the forests on its northern slope, and some of its peaks are from 3000 to 3500 ft high.
The limestone and sandstone foot-hills are clothed with vines and fruit trees, and are broken by deep alluvial valleys of particular fertility. With the exception of a small portion of the Montagne Noire, which drains into the river Aude, the whole department belongs to the basin of the Garonne. The eastern portion of the department has the climate of Auvergne, the severest in France, but that of the plain is Girondin.
Tarn is bounded north and east by Aveyron, southeast by Hérault, south by Aude, southwest and west by Haute-Garonne, northwest by Tarn-et-Garonne. The slope of the department is from east to west, and its general character is mountainous or hilly.