Expanded substantially in the first half of the 12th century, it eventually covered large portions of Satsuma, Ōsumi and Hyūga Provinces.

In 1185, Koremune no Tadahisa was appointed as jitō of the estate, and adopted the name of the manor, thus founding the Shimazu clan.

On January 1, 2006, the towns of Takajō, Takazaki, Yamada and Yamanokuchi (all from Kitamorokata District) were merged into Miyakonojō.

Miyakonojō has a mayor-council form of government with a directly elected mayor and a unicameral city council of 29 members.

In terms of national politics, the city is part of the Miyazaki 1st district of the lower house of the Diet of Japan.

Miyakonojō is also famous for producing traditional Japanese archery bows from the abundant bamboo in the area.