Poa mannii is a perennial grass growing between 50 and 75 cm tall. The stems have short rhizomes that extend incrementally in a single direction. The culms are tufted, strongly compressed, wiry, erect, glabrous, and striate, with a somewhat glaucous appearance. The internodes are solid. The leaves are simple and alternate, with flat, lax blades measuring up to 15 cm long and 2–4 mm wide. The apex is acuminate, and the base is somewhat clasping. The upper surfaces are scabrous, while the lower surfaces are glabrous. The lower leaf blades are deciduous from the sheaths. The veins are parallel. The sheaths are closed, glabrous, striate, and usually shorter than the internodes. The auricles are crescent-shaped, and the ligule completely encircles the culm, measuring approximately 0.5 mm long. The ligule is membranous except on the side opposite the blade, where a sheath nerve projects as a prominent tooth 2–4 mm long. Stipules are absent. The flowers are arranged in ovoid panicles that are usually less than 5 cm long. The primary branches measure 0.5–2 cm long. The spikelets (pictured) arise in the upper half of the branches and are pale greenish or yellowish brown. Each spikelet is (2-)4-5-flowered, flattened, and 4–7 mm long. The rachilla elongates between the florets and is subglabrous. The glumes are slightly unequal, narrow, approximately 3 mm long, 3-nerved, glabrous, with an acuminate apex. The lemmas are 3–4 mm long, with cobwebby hairs at the base. The keel and lateral nerves are scabrous-ciliate, while the intermediate nerves are not prominent. The apex is acute. The palea is lanceolate to narrowly ovate, measuring 3–3.5 mm long, 2-nerved, and keeled. The keels are scabrous-ciliate, the margins are hyaline, and the apex is acute or bifid. The fruit is a reddish-brown caryopsis, ellipsoid to fusiform, slightly trigonal, and approximately 1.5 mm long. It is found growing on cliffs and rock faces between 460-1,222 meters.