The shell shape and sculpture are relatively variable in the analysed individuals across the species' distribution. The species exhibits a conical morphology, oblique with a very thin and translucent shell. The anterior part of the animal is laterally compressed. It has a medium shell size (maximum length 90 mm) and a medium profile. The apex is situated at the anterior 10% of the shell's length, anteriorly directed and curved toward the border. The anterior slope of the shell is concave, while the posterior and lateral ones are convex. The aperture of the shell is oval to oblong. The surface of the shell has a sculpture of a few radial ribs, more obvious in the anterior zone, and concentric growth lines, which increase in thickness towards the shell margin. The margin of the shell is relatively even. The external coloration varies (pearly cream, yellow and light brown with darker tones towards the margin with a brown/red apex). The internal part of the shell is simple with white nacreous coloration. External anatomy: The ventral area of the foot varies from dark grey to black and the epipodial fringe is highly recognizable. The mantle fold is narrow and cream-coloured. The mantle tentacles are in alternated series of three white shorter ones and a white longer one. Nevertheless, some individuals also exhibit a configuration of three shorter (white/black/white) and a black longer one. The cephalic tentacles are dorsally pigmented with a black line.

Shell rather small, up to 43 mm. (1,625 inches) in length, elliptical, thin and fragile, with the apex almost at the anterior end. Usually the surface is almost smooth, but occasionally moderate radial ridges are developed, as well as corrugations around the anterior margin. Colour greenish olive to light brownish, sometimes reddish bronze at the apex, and the interior is silvery iridescent. Length 27-43 mm; Width 18-26 mm; Height 7,5-18.0 mm

The previously recognized distribution of N. mytilina included the Magellanic province and the Kerguelen Archipelago as well as Antarctica. Cantera & Arnaud (1985) treated N. kerguelenensis from the Kerguelen Islands as a junior synonym of N. mytilina. However, recent multilocus molecular analyses have demonstrated that kelp-associated N. mytilina-like individuals from the Kerguelen Islands represent a particular morphotype of N. kerguelenensis, which is genetically distinct from N. mytilina (Gonzalez-Wevar et al., 2017). At the same time, no confirmed record exists for N. mytilina in Antarctica or in peri-Antarctic islands, such as South Shetlands Islands or South Orkneys. Accordingly, the distribution of N. mytilina is restricted to southern South America and the Falkland/Malvinas Islands. As recorded for other Patagonian marine invertebrates, N. mytilina exhibits population-level genetic differentiation between Pacific Patagonia and the Falkland/Malvinas Islands.

Range: Southern Chile, Straits of Magellan, Tierra del Fuego, Falkland Islands and Kerguelen Island.

Magellanic province. Pacific Patagonia: from Guarello Island (50°S) to Cape Horn (56°S). Atlantic Patagonia: Tierra del Fuego to Puerto Deseado (47°S). Falkland/Malvinas Islands.
Habitat: Nacella mytilina is regularly found on macroalgae (Macrocystis, Gigartina and Lessonia; Rosenfeld et al., 2015) but in some localities individuals can also be found on inter- and subtidal rocks.