Thick-stemmed epiphyte on tree trunk; stems with mucilaginous, reddish-brown latex; internodes short, c. 5 cm diam., drying dark brown; cataphylls 2-ribbed, c. 42 cm long, drying dark brown, deciduous. Leaves large; petiole terete with tiny ridges, sulcate at the base, 67–99 cm long, c. 1.5 cm diam., drying dark brown, sheathing to c. 6 cm; blade ovate-sagittate, 84–99 cm long, 60–74 cm wide, 1.3–1.4 × longer than wide, 1–1.3 × as long as petiole, abruptly long-acuminate at apex, conspicuously lobed at base, subcoriaceous, drying dark grey-green above and light grey-green below, matte on both sides, often ripping apart along some minor veins upon drying; anterior lobe 46–69 cm long, broadly convex along the margins; posterior lobes 20–30 cm long, 21–31 cm wide, extending inward and sometimes overlapping; sinus broadly spathulate, 21–31 cm deep, 6–9 cm wide; midrib prominent, flattened and slightly lighter than the blade above, rounded and dark yellow-brown below; primary lateral veins 5–6 pairs, arising at 60–85º angle, flattened above and rounded below, concolorous with midrib; basal veins 9 pairs, the 1st pair free to the base, the remaining fused to various extent and branching at various intervals on both sides of the posterior rib; posterior rib thick, free for c. 7 cm and then extending and branching inside the posterior lobe; minor veins numerous and conspicuous, arising from both the midrib and the primary lateral veins and forming a dense network of closely parallel finely raised veins covering the surface of the blade. Inflorescences 3 per axil; peduncles 10–14 cm long, 4–5 mm diam., drying dark brown; spathe 10–15 cm long, 2.5–3.5 cm diam., tube red, blade white; spadix 12–13 cm long, slightly shorter than spathe, green or white; staminate portion 5–6 cm long, 1–1.5 cm diam.; sterile portion 0–1.2 cm long, 6–8 mm diam.; pistillate portion 5.5–7.5 cm long, 2–2.5 cm diam.; pistils 4.5–5 mm long, 3.5–4 mm diam., stigma 1.5 mm wide, 0.3 mm thick; locules 5, ovules 5–8 per locule with axile placentation, branching from a single point on the midportion of the locule, 0.15 mm long; seeds cylindrical, 1–1.5 mm long, constricted near the apex.
The species is a member of Philodendron subg. Philodendron sect. Philodendron subsect. Philodendron series Philodendron and is characterised by its epiphytic habit, short thick internodes, 2-ribbed, deciduous cataphylls, terete petioles with tiny ridges and drying dark brown, large ovate-sagittate matte blades, which dry grey-green above with finely raised veins and grey-green below with dark yellow-brown major veins on lower surface, as well as by its clusters of up to 3 inflorescences per axil with green or red and white spathes.
Philodendron mcphersonii somewhat resembles P. zhuanum Croat of Panama, but that differs by its larger and broader leaves (blade length/width ratio of 1.3–1.4 for P. mcphersonii versus 1.8–2 for P. zhuanum), its strong posterior rib naked to 7 cm, and its somewhat smaller inflorescences.