Adult males measure 18–24 mm (0.7–0.9 in) and adult females 22–32 mm (0.9–1.3 in) in snout–vent length. The appearance is stocky.^[4] The head is broader than it is long. The snout is rounded to elliptical. The tympanum is obscure. The finger and toe tips bear fleshy fringes and broad, oval discs; the fingers have rudimentary webbing while the more heavily webbed. The dorsal ground colour is pale grey to dark chestnut; the pattern is highly variable and may include bars, stripes, and mottling, or be plain. The iris is brown or goldish and has a horizontal dark bar.^[5]

Philautus mjobergi occurs in submontane forests^[4] and montane (oak-chestnut) forests^[1] at elevations of 900–3,000 m (3,000–9,800 ft) above sea level.^[4] Males call at night from the shrub layer 0.15–3 m (0.5–9.8 ft) above the ground. The eggs may be laid in pitcher plants.^[1][5]

This species may not be threatened because most logging occurs at lower elevations,^[1] although this statement is based on a higher lower limit for this species (1500 m^[1]) than found in other sources (800 m^[4]). It occurs in several protected areas, including Kinabalu Park and Gunung Mulu National Park.^[1]