Warnstorfia exannulata grows as an interwoven mat of glossy moss. It is yellow-green when young and matures to be dark-green, purple-brown, or reddish-brown. W. exannulata can have either regular or irregular pinnate branching and has secondary pinnate branching. Stems are often curved. The main stems can grow up to 30 cm in length depending on growing conditions, with one study finding their longest sample at 28 cm aged approximately 8 years old. The stem and branch leaves are similar in shape with the branch leaves being slightly thinner on average. Leaves are linear-lanceolate in shape and 3-4mm in length gently tapering to a point. The shape of the leaf is varying between straight to falcate. The base of stem leaves is very slightly decurrent along the stem, which can be an identifying trait of this species in comparison to other Warnstorfia species. It has a strong single costa that extends up two thirds of the leaf in both stem and branch leaves. The alar region is very clearly differentiated and makes two triangular regions of 1-3 rows of cells at the base of the stem leaf. The alar region is made up thin walled rectangular shaped cells that are distinctive from the lamina cells which are more elongate in shape A cross section of stem shows a characteristic double layer of cortical cells that have thick walls and central medullary cells are more thinly walled.
