Shell of medium size for genus, adult size up to 14.8 mm; fusiform, elongate, slender. Suture slightly impressed. Protoconch paucispiral, 1.5 inflated whorls, broader than early teleoconch whorls. Teleoconch of 7-7.5 whorls. Axial sculpture of fine growth lines only. Spiral sculpture very weak, consisting of weak subsutural cord and basal cords ventrally ranging to adapical edge of columella. Outer lip slightly thickened, smooth, bearing only continuation of basal cords. Inner lip smooth. Columellar callus marked by distinct rim along siphonal canal. Parietal callus slightly thickened. Columella smooth. Siphonal short, open, slightly recurved. Teleoconch whorls off-white in colour. Protoconch semi-translucent, off-white, with brown nucleus. Aperture and columella off-white. Operculum typical of genus, pale brown, semi-translucent, with terminal nucleus. Holotype height 10.6 mm.

M. gracilis clearly belongs to the longissima group, in which it is closest to M. longissima and Mitrella praetermissa. These species all have a paucispiral protoconch, but the protoconch of M. gracilis is much more swollen. Other differences include the larger size, the presence of a subsutural cord and the straighter teleoconch whorl profile in M. gracilis.

New Caledonia: between Grande Terre and Ile des Pins, Norfolk Ridge, New Hebrides Ridge, 300-447 m, alive in 305-310 m.