Uasin Gishu County until 1750s was historical a savannah plateau with plenty of wildlife such as Lions, Leopards, Hyenas among others. The Maasai arrived in early 1600 and displaced the Sirikwa people occupied the Plateau and named it as Uasin Gishu after the Illwuasin-Kishu Maasai clan which was a grazing area of the clan. The Nandi displaced Maasai from the Plateau and occupied the Plateau as their grazing areas. Upon the arrival of the British, the Nandis were pushed to the Nandi African reserves while others as laborers in their farms. The plateau then was Anglicized as Uasin Gishu by the British.

In 1903, the Eldoret area was proposed as a potential Jewish homeland, as the British Uganda Programme. This proposal was rejected by the British Jewish community leaders. In 1908, fifty eight families of Afrikaans-speaking South Africans settled in Uasin Gishu plateau. They were followed by sixty more families in 1911 and more later. The town of Eldoret was established in the midst of the farms they created.

After independence in 1963, the County was administered by the Municipality of Eldoret and the Wareng County Council. In 2010, it was enacted as a one of the 47 devolved Units of Kenya and elected its first Governor, H.E Jackson Kiplagat araap Mandago, EGH in 2013. And in 2022 H.E Hon. Jonathan Bii Chelilim was elected as the second Governor .