The administrative centre was located at Brensholmen on the western shore of the island of Kvaløya.

On Senja island, Hillesøy Municipality included the larger villages of Fjordgård, Husøy, and Botnhamn.

If this is the case, then the first element would be derived from the word hellir which means "rock slab" or "cave".

On 6 January 1908, a royal resolution changed the spelling of the name of the municipality to Hillesøy, to give the name a more Norwegian and less Danish spelling due to Norwegian language reforms.

It was part of the Hillesøy prestegjeld and the Trondenes prosti (deanery) in the Diocese of Nord-Hålogaland.

The highest point in the municipality was the 942-metre (3,091 ft) tall mountain Keipen on the island of Senja.