[1] Its scientific name derives from its discoverer, Magnus Earle Johnson, Three Kings Islands explorer and captain of the yacht Rosemary, who landed botanists on the islands on several occasions.

[2] Sargassum johnsonii grows in rocky intertidal habitats, often extending to the subtidal zone up to a depth of 40 m. It is a foundation species, and being very abundant around Three Kings Islands it creates numerous microhabitats for other forms of marine life such as bryozoans, sea squirts, and sponges.

Basal leaves have strap-shaped form, simple or lobed with an indistinct midrib.

The thallus is dark brown, with a tough, leathery texture.

It has no air-vesicles, and its receptacles are small, swollen, and around 2 mm long, clustered in the axils of the upper leaves.