It occupied the south-western corner of the North Island, extending south from the Waikanae River and lying to the west of the summits of the Rimutaka Ranges.
The county's name arose from the fact that a large amount of its land area lay in the Hutt River catchment.
The county initially had 8 ridings: Belmont, Epuni, Horokiwi, Makara, Mungaroa, Porirua, Wainui-o-mata and Whareroa.
[2] During its time, several towns, boroughs, districts, and eventually cities developed within the county.
The boroughs of Melrose, Onslow, Karori, Miramar, Johnsonville and Tawa Flat initially developed in parts of Hutt County just outside Wellington.