The area of Aargau and the surrounding areas were controlled by the Helvetians, a member of the Celts, as far back as 200 BC, eventually being occupied by the Romans and then by the 6th century, the Franks. The Romans built a major settlement called Vindonissa, near the present location of Brugg.

The reconstructed Old High German name of Aargau is Argowe, first unambiguously attested (in the spelling Argue) in 795. The term described a territory only loosely equivalent to that of the modern canton, including the region between Aare and Reuss, including Pilatus and Napf, i.e. including parts of the modern cantons of Bern (Bernese Aargau, Emmental, parts of the Bernese Oberland), Solothurn, Basel-Landschaft, Lucerne, Obwalden and Nidwalden, but not the parts of the modern canton east of the Reuss (Baden District), which were part of Zürichgau.

Within the Frankish Empire (8th to 10th centuries), the area was a disputed border region between the duchies of Alamannia and Burgundy. A line of the von Wetterau (Conradines) intermittently held the countship of Aargau from 750 until about 1030, when they lost it (having in the meantime taken the name von Tegerfelden). This division became the ill-defined (and sparsely settled) outer border of the early Holy Roman Empire at its formation in the second half of the 10th century. Most of the region came under the control of the ducal house of Zähringen and the comital houses of Habsburg and Kyburg by about 1200.

In the second half of the 13th century, the territory became divided between the territories claimed by the imperial cities of Bern, Lucerne and Solothurn and the Swiss canton of Unterwalden. The remaining portion, largely corresponding to the modern canton of Aargau, remained under the control of the Habsburgs until the "conquest of Aargau" by the Old Swiss Confederacy in 1415. Habsburg Castle itself, the original seat of the House of Habsburg, was taken by Bern in April 1415. The Habsburgs had founded a number of monasteries (with some structures enduring, e.g., in Wettingen and Muri), the closing of which by the government in 1841 was a contributing factor to the outbreak of the Swiss civil war – the "Sonderbund War" – in 1847.

Swiss History Timeline - Aargau
Aargau coat-of-arms