Ascidia mentula has a tough leathery envelope or tunic composed partly of cellulose.

The translucent tunic encloses a fluid-filled body with irregular bulges and two inconspicuous siphons.

[2] It favours crevices and shady gullies and sometimes adheres to Laminaria holdfasts, stones and shells.

[4] Ascidia mentula feeds on plankton that it filters from seawater with a mucous net.

Fertilization is external and the tadpole larvae spend a short time in the plankton before settling on the seabed, undergoing metamorphosis and becoming juveniles.