It has a thin eel-like body around 12 to 15 cm (4.7 to 5.9 in) long, with two low dorsal fins on the back half.

Unlike M. mordax, M. praecox has eyes that appear to have the potential for dichromatic vision despite being similarly adapted for low-light environments.

[2] For many years M. praecox was known only known from the Moruya and Tuross rivers of southern New South Wales.

[1][3] Two geographically isolated populations of non-parasitic Mordacia found in the Noosa river region of Queensland are believed to represent one or two closely-related, but distinct, species that are yet to be described.

Unlike most other species of lampreys, M. praecox spends its entire life in fresh water.