Northern Fur Seals exhibit extreme sexual dimorphism, with males being 30-40% longer and more than 4.5 times heavier than adult females. Males can be as large as 2.1 m and 270 kg. Females can be up to 1.5 m and 50 kg or more. Newborns weigh 5.4-6 kg, and are 60-65 cm long. Pups are blackish at birth, with variable oval areas of buff on the sides, in the axillary area and on the chin and sides of the muzzle. After 3-4 months, pups moult to the colour of adult females and subadults. Northern Fur Seals become sexually mature at 3-5 years old, at which time females usually produce one pup a year for most of the rest of their lives. Gestation lasts 51 weeks, which includes a delay of implantation of 3.5 to 4 months. Longevity of this species is 25 years (Reijnders et al 1993). Males do not become physically mature, and large enough to compete for a territory that will be used by females, until they are 8-9 years old. The generation time for females is approximately 12-15 years depending on the determination methods used.

Breeding on the Pribilof Islands occurs from mid-June through August, with a peak in early July (the median date in southern California is approximately 2 weeks earlier than at the Pribilofs). This is a highly polygynous species. Males arrive at the rookeries up to one month before females and vocalize, display and fight to establish and maintain territories.

Northern Fur Seals usually give birth a day after arrival at the rookery. Mean time from birth to oestrous is 5.3 days, followed by a departure for a mean of 8.3 days for the first feeding trip. Females breeding at the Pribilof Islands are located relatively far from the foraging areas, which are concentrated at the edge of the continental shelf and hence females in this population consistently make longer foraging trips than most other female otariids, with a mean trip length of 6.9 days. Once foraging begins the mean depth of dives is 68 m and average duration is 2.2 minutes with maximum depth recorded of 207 m and maximum duration of 7.6 minutes. Pups are visited 8-12 times over the lactation period and attended for a mean of 2.1 days during each visit, before being abruptly weaned at 4 months old.

Northern Fur Seals are one of the most pelagic pinnipeds. They spend most of the year at sea, rarely (if ever) returning to land between one breeding season and the next. Thus, males spend an average of only 45 days ashore a year and females only 35 days a year. Once weaned, juveniles go to sea and do not haul-out until they return, usually to the island of their birth, 2-3 years later. At sea, Northern Fur Seals are most likely to be encountered alone or in pairs, with groups of 3 or more being uncommon. They forage relatively far from shore, over the edge of the continental shelf and slope. Diving is concentrated around dawn and dusk. Northern fur seals spend quite a bit of time rafting at the surface, either asleep or grooming. They employ a wide variety of resting postures, including raising one or more flippers into the air, and draping one of their fore flippers over both of the rear flippers to form a posture known as the "jug handle" position.

Many animals, especially juveniles, migrate from the Bering Sea south to California or the waters off Japan, to spend the winter feeding.

The diet varies by location and season and includes many varieties of epipelagic and vertically-migrating mesopelagic schooling and non-schooling fish and squid. Prey species of importance in the waters off California and Washington include anchovy, hake, saury, several species of squid and rockfish, and salmon. In Alaskan waters, Walleye Pollock, Capelin, Sand Lance, Herring, Atka Mackerel, and several species of squid are important prey.

Predators include Killer Whales, sharks, and Steller Sea Lions.