The interior of the county encompasses the traditional district of Setesdal, through which the river Otra flows to the coast.

In 2017, the Parliament of Norway voted to merge Aust-Agder and Vest-Agder counties into one large region, Agder, effective 1 January 2020.

[4] The county was part of the Aust-Agder District Court and the Church of Norway Diocese of Agder og Telemark.

The meaning of the name is "(the) eastern (part of) Agder", since the word aust is the Nynorsk form of "east".

The amt was named after the old Nedenes farm (Norse Niðarnes), since this was the seat of the amtmann (County Governor).