The thallus of U. mammulata is usually 4 to 15 cm (1.6 to 5.9 in) in diameter, but specimens have been known to reach 63-centimetre (2.07 ft) in the Smoky Mountains of Tennessee.

This species is found on boulders and steep rock walls in forests and around lakes.

[3] Like most lichens, U. mammulata is sensitive to air and water quality.

If conditions are optimal, seeing rocks or cliffs covered in dinner plate-sized thalli is not unusual.

[2] However, it has been suggested that U. mammulata is not as sensitive to pH and water quality as it is to the frequency and duration of precipitation.