Archaeomarasmius is known from two fruit bodies (or mushrooms) recovered from amber, one consisting of a complete cap with a broken stem, the other consisting of a fragment of a cap. The cap has a diameter ranging from 3.2 to 6 mm (0.13 to 0.24 in), while the stem is 0.5 mm (0.02 in) thick. Spores were also recovered from the amber, and are broadly ellipsoid to egg-shaped, measuring roughly 7.3 by 4.7 μm.
Archaeomarasmius is thought to be saprobic. This means that obtained nutrients by breaking down organic matter. It likely fed on plant litter or other forest debris.