Freckle pelt is common on the forest floor, in thickets, and in tundra. This foliose species grows horizontal to the ground, the lobes upturned at the edges, showing the white undersurface. The thallus can grow in rosettes up to 10 or 15 cm across in lush habitats, but is usually smaller. The upper surface is a bright grassy green (paler grey-green if dry, sometimes sunburnt brown), with dark flecks. The flecks (cephalodia) are pockets of cyanobacteria, a nitrogen-fixing bacterium. The presence of cyanobacteria allows access to important nutrients. The underside of the thallus lacks a cortex, making it soft-textured, and darkens abruptly towards the center. This lichen frequently has apothecia, which are rounded, dark brown to black, extended at the edge of the thallus. There are many other species of Peltigera in Denali, but most lack cephalodia. Closely related and abundant Peltigera britannica is highly similar, but the cephalodia sit on the surface of the thallus and can be easily removed with a thumbnail.
Peltigera aphthosa reproduces both sexually and asexually. The fruiting bodies are apothecia.