The county was established in 1415 after the Swiss conquest of the Aargau and was ruled as a shared condominium until 1798 when it became part of the short-lived Canton of Baden.

[1] On 16 November 1414, Emperor Sigismund called the Council of Constance to settle the Western Schism between the three popes (Benedict XIII, Gregory XII, and John XXIII), all of whom claimed legitimacy.

Under Habsburg rule, Aargau was divided into multiple sections (German: Ämter), which were maintained under the Confederation.

The county included the former Ämter of Baden and Siggenthal, the Bishop of Constance's vogtei of Klingnau, Zurzach and Kaiserstuhl as well as the parish of Leuggern on the left side of the Aare.

It was divided into eight inner (Rohrdorf, Birmenstorf, Gebenstorf, Dietikon, Wettingen, Siggenthal, Ehrendingen, Leuggern) and three outer districts (Klingnau, Zurzach, Kaiserstuhl), which included the parishes of Kadelburg, Lienheim and Hohentengen on the right bank of the Rhine.

All these overlapping jurisdictions caused numerous conflicts, but gradually the Confederation was able to acquire these rights in the county.

Together with the courts, the three administrative centres had considerable local autonomy but were ruled by a governor who was appointed by the Acht Orte every two years.

However, customs fees brought in so little that in the 17th century, the right to collect duties was auctioned off to the highest bidder.

While the rural upper class tried several times to finally expel the Jews, the financial interests of the authorities prevented this.

With an ever-changing administration, the county lacked a coherent long-term economic policy or support for reforms.

The governors were appointed from both Catholic and Protestant cantons and since they changed every two years, neither faith gained a majority in the county.

On 19 March 1798, the governments of Zurich and Bern agreed to the creation of the short-lived Canton of Baden in the Helvetic Republic.

After World War II, this formerly agrarian region saw striking growth and became the district with the largest and densest population in the canton (110,000 in 1990, 715 persons per km2).