South American Fur Seals are sexually dimorphic. Adult males are approximately 1.3 times longer and 3.3 times heavier than adult females. Adult males reach 1.9 m and 120–160, and possibly 200 kg; females are about 1.4 m long and weigh 40-50 kg. Newborns are 60-65 cm and 3.5-5.5 kg. Most females give birth for the first time when they are 4 years old. Pups are born shortly after females return to the colonies. Oestrous occurs 7–10 days later, and following mating, a female begins to make foraging trips punctuated by time attending the pup ashore. Pups are weaned at 8 months to 2 years of age. Females will nurse a yearling and newborn pup at the same time.

Breeding takes place from mid-October through mid-January. The timing of breeding may differ in the Peruvian and Chilean-Atlantic colonies. Colonies are generally found along rocky coasts, on ledges above the shoreline or in boulder strewn areas. Most areas utilized have some source of shade such as at the base of cliffs or under boulders and easy access to the ocean or tidal pools. Males are polygymous and territorial, and fighting can result in dramatic wounds and scars. Individual bulls can occupy territories for up to 60 days and have up to 13 females on their territories at Uruguayan colonies. Male vocalizations include a bark or whimper, a guttural threat, and a submissive call. Females growl and have a pup-attraction call that is a high-pitched wail.

Time spent on trips and attending the pup likely varies with location and changes in marine productivity; El Niño years have a negative impact on animals in Peru and during them females must spend much more time attempting to forage. Female attendance in Uruguay is affected by weather with females spending less time ashore during the day when ground temperature exceeds 36ºC and more time ashore during storms. Survival rates of pups can be quite low when marine productivity is low and storm surges can sweep large numbers of pups off colonies. Locally, pup morality inflicted via predation by adult male South American sea lions can be significant at some colonies. Data collected on adult female South American fur seals during an El Niño event resulted in mean dives to 29 m, with a maximum of 170 m and mean duration of 2.5 minutes and maximum dive length of 7 minutes.

The population along the coast of Patagonia is linked to the Uruguayan colonies. Very few births occur in the Patagonian population, so the recent increases in numbers in this area are almost certainly due to a migration from the Uruguayan islands. It is possible that a similar movement occurs between the Staten Island colony and the Chilean colonies via the Beagle Channel.

At sea, these fur seals may be seen travelling or rafting at the surface in groups. South American Fur Seals will "porpoise,” or leap clear of the water when moving rapidly at sea, sometimes travelling like this in large groups. While resting at the surface they spend considerable time grooming and assume many poses typical of southern fur seals, including waving both hind flippers in the air while the head is submerged. Groups often form in the water at the base of a colony. They are frequently seen grooming while resting at the surface.

Demersal and pelagic fishes make up the majority of the diet in Uruguay and include: Anchoveta, weakfish, cutlassfish, and anchovy. Cephalopods, lamellibranchs and gastropods are also taken. Additional prey taken in other areas includes sardines, mackerel, hakes and crustaceans such as lobster krill in southern Chile and the Falkland Islands where squid is also a common prey item.