[6] Lejeunea mandonii is endemic to Europe where it can be found within the following countries and territories: United Kingdom, Ireland, Portugal, Canary Islands and Madeira.

[4] Lejeunea mandonii can also be found growing on rocky overhands, ravines and north facing cliffs near to the coast.

[5] Non-native evergreen tree species such as the Western Red Cedar Thuja plicata when planted in proximity to L. mandonii causes heavy shade.

Regular human intervention is needed to cut back the shrub so that it does not shade out rocky habitat where Lejeunea mandonii grows.

[6][5] Due to the arrival of the fungi Hymenoscyphus fraxineus within the United Kingdom, they are threatened by Ash dieback disease.