With an overall length of 800 to 1000 mm, this is a large and robust mongoose. It has a mass of between 2.5 to 4.2 kg. Their tails are 300-410 mm long. Their colour is a uniform dark brown with a lighter nose. Their bodies are relatively short the tail is covered with long, shaggy hairs.
Ears are adpressed to the head. They have a large skull with a short muzzle. The large brain is associated with increased tactile sensitivity and muscular control of the forepaws. These characteristics enables this semi-aquatic mongoose to locate and eat crabs and other relatively hard-bodied prey.
Their teeth are adapted to crush the hard outer shells of shell fish, rather than to shear. This is the only herpestid with unwebbed feet which splay on soft substrates. This trait results in a very distinctive spoor.
Females give birth to two young per litter and more than one litter can be produced per season. Young start taking food after one month, their eyes and ears open after 10 days. Little is known about their sexual behaviour, but it is known that they breed twice a year, once in the middle of the dry season and once in the rains.
The female prepares a nest of dry grass in a hole. If there are no holes available in swampy areas, the young are raised on a nest of reeds, grass, and sticks. Up to three young per litter have been recorded. Sometimes a second adult also accompanies the family. The young usually are weaned and depart from their birthplace in a few months.
They are solitary in habits, a semi-aquatic and nocturnal species. Marsh mongooses are regular in their habits and follow pathways that are smooth and well-defined. The pathways tend to follow rivers or shorelines, often hidden by tall grass and reed clumps. To catch a bird, the mongoose lies on its back and looks as if it's sunbathing.
In this position, the pale, pink anal area assumes a startling prominence against the surrounding dark fur. This display is claimed to induce birds to approach and peck at the anus, whereupon the mongoose seizes the bird. When approached by a threatening presence, the mongoose makes a low growl, which may be reinforced by sudden explosive barking growls in a deeper tone.
When the mongoose is cornered or distressed, it ejects jets of foul brown fluid from its anal sacs. Mongooses make a high-pitched cry and an open-mouthed bleat when excited. They are frequently seen singly. They are highly territorial, and their territories are spaced along their linear habitat.