Cap 2-8 mm broad, at first hemispherical, expanding to broadly-conic, occasionally with a low umbo, convex at maturity, not hygrophanous; margin weakly sulcate, striate to near the disc, incurved in youth, then decurved, sometimes slightly raised in age; surface reddish-orange shading to yellow-orange or pale-yellow at the margin; surface inconspicuously white-pruinose early in development; context thin, approximately 0.5 mm thick, creamy-yellow to pale yellow-orange; odor and taste not distinctive.
Gills adnate to notched, fairly well spaced, at first pinkish-cream, in age cream- yellow; edges even, not marginate; lamellulae in two to three series.
Stipe 1.0-5.0 cm in length, 0.5 mm in width, equal, round, hollow, slightly viscid, white-pruinose over a translucent lemon-yellow ground color, becoming whitish, sparsely pruinose near the base, the latter with anchoring pallid hairs; partial veil absent
Spores 8.5-11.5 x 3.0-4.0 µm, narrowly elliptical to subfusoid in face-view; in profile narrowly elliptical, inequilateral with a straight and curved side; smooth, thin- walled, hilar appendage conspicuous, inamyloid; spore deposit not seen, presumably white.
Solitary to scattered in moist, shaded habits; fruiting on hardwood and conifer litter, sometimes among mosses and liverworts, but not attached; Coast Ranges and Sierra Nevada; fruiting from late fall to mid-winter along the coast, fall and spring in the mountains; fairly common.
Compared to the dull hues of most Mycenas, Mycena acicula is a visual treat, the tiny caps bright reddish-orange, complemented by a yellowish stipe. What adaptive value the brilliant coloration conveys if any, is unclear, but it is a character shared by a small number of Mycenas. These include, Mycena oregonensis, M. aurantiidisca, M. adonis, and M. strobilinoides. Mycena oregonensis differs in possessing a more yellowish to yellow-orange cap, the stipe yellow-pruinose, dry, not viscid. Mycena aurantiidisca can usually be distinguished by a reddish-orange cap that fades to white at the margin. Mycena adonis as interpreted here has a cap with pinkish tones, i.e. pinkish-red, rather than reddish-orange and a pink to white stipe. Finally, Mycena strobilinoides, which mimicks M. acicula with a bright reddish-orange cap, is distinctly larger, up to 2.0 cm broad, has amyloid spores unlike the above species, and is presently known only from under conifers in northwestern California. Molecular studies suggest that these brightly colored species may not belong in Mycena and at some point could be transferred to new or other genera.