It was formed in 1936 from parts of the conquered Ethiopian Empire following the Second Italo-Ethiopian War with the capital was Jimma.

In November 1938 some territory of Galla-Sidamo in the Scioa region was given to the neighboring Addis Abeba Governorate, enlarging it to the Scioa Governorate.

The area bordering the Anglo-Egyptian Sudan was the stronghold of the Ethiopian resistance against the Italians until 1939, when it was practically pacified.

The Governorate of Galla-Sidamo was subdivided into:

Main colonial governors: Agenore Frangipani; Guglielmo Nasi; Enrico Cerulli; Pietro Gazzera; Luigi Frusci; Alessandro Pirzio Biroli