Northern Fur Seals exhibit extreme sexual dimorphism, with adult males measuring 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. Adult females can measure 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 color on the sides, in the axillary area, and on the chin and sides of the muzzle. After three to four months, pups moult to the colour of adult females and subadults. Northern Fur Seals become sexually mature at three to five years old, at which time females can produce one pup each year for most of the rest of their lives (Scheffer and Wilke 1953, Scheffer 1962). Kuzin (1999) noted that at times of rapid population growth on Tuleny Island, some females were fertilized as early as age two with five year old females achieving a pregnancy rate of 80%. Gestation lasts 51 weeks, which includes a delay of implantation of 3.5-4.0 months (York and Scheffer 1997). Females may produce pups up to age 23 (Lander 1981). Males do not become physically mature, and large enough to compete for a territory that will be used by females, until they are eight to nine years old. The generation time for females is approximately 12-15 years depending on the determination methods used. Pacifici et al. (2013) estimated the generation time at 14.1 years.

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 two 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 estrous 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 of Alaska 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 eight to 12 times over the lactation period and attended for a mean of 2.1 days during each visit, before being abruptly weaned at four months old.

Northern Fur Seals are one of the most pelagic pinnipeds. They spend most of the year at sea, and 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, two to three years later. At sea, Northern Fur Seals are most likely to be encountered alone or in pairs, with groups of three 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. Many animals, especially juveniles, migrate from the Bering Sea south to California or the waters off Japan, to spend the winter feeding. (Sterling et al. 2014).

The Northern Fur Seal diet varies by site, oceanic domain 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, Salmon, and several species of Squid and Rockfish. On the Pribilof Islands, prey species reflect foraging patterns on and off the continental shelf. Satellite telemetry and stable isotope analysis indicate that those seals foraging on the shelf use typical on-shelf species like Walleye Pollock, Pacific Herring and gadid species while off shelf foragers included species such as myctophids. Bogoslof Island in the Aleutian Islands of Alaska is a volcanic island surrounded by deep water and the most common prey species there included Northern Smoothtongue and gonatid squid species (Zeppelin and Orr 2010).

Predators include Killer Whales, Sharks and Steller Sea Lions (Gentry 1998).

Northern Fur Seals are a widely-distributed pelagic species in the waters of the North Pacific Ocean and the adjacent Bering Sea, Sea of Okhotsk and Sea of Japan. They range from northern Baja California, Mexico, north and offshore across the North Pacific to northern Honshu, Japan. The southern limit of their distribution at sea is approximately 35°N (Rice 1998). Vagrants reach the Yellow Sea in the west and eastern Beaufort Sea to the north. They breed on rookeries in Russia (Kuril Islands, Commander Islands and Tuleny Island (Kuzin 1999), Alaska (Pribilof Islands and Bogoslof Island) and California (San Miguel Island) (Loughlin et al. 1994). In the past, the vast majority of the population bred at rookeries on the Pribilof Islands, with substantial numbers on the Commander Islands. However, a significant population decline on the Pribilofs concurrent with the establishment of a new breeding site in the eastern Aleutian Islands and population increases in the Kuril Islands of Russia has resulted in the Pribilof Islands now representing approximately 45% of the global population.

Following the termination of the Interim Convention on the Conservation of the North Pacific Fur Seal in 1984, the Northern Fur Seal is now managed on land independently by the Russian Commonwealth of Independent States and the United States. The eastern north Pacific stock of the Northern Fur Seal was listed as depleted under the U.S. Marine Mammal Protection Act in 1988, and a final conservation plan was completed in December 2007.