Guadalupe Fur Seal are sexually dimorphic, with males 1.5-2 times longer and approximately 3–4 times heavier than adult females. Adult males may reach 2 m. Two adult males that were measured were approximately 1.8 and 1.9 m in length. The 1.9 m male was estimated to weigh 160-170 kg. Adult females average 1.2 m and reach approximately 1.4 m, and weigh 40-50 kg. Pups are estimated to be 50–60 cm long, weigh around 6 kg at birth and are weaned when 9-11 months.

Guadalupe Fur Seals are polygynous, with males establishing territories that are occupied by an average of six females. Pups are born from mid-June to August with a median birth date of 21 June. Male tenure on territories lasts at least as long as 31 days. Males defend territories with vocalizations, displays, and mutual displays with neighbouring bulls. Fighting between males is rare once territories are established. Females select only male territories that provide cover and shade from the sun and all territories occupied by females are fronted by water and include tidal pools. Adult females enter the water daily, presumably for cooling, while otherwise “ashore” attending their pups. Most animals breed in small caves, grottos, and cliff and boulder areas on the rugged east coast of volcanic Guadalupe Island. A small breeding colony was discovered on the east side of the easternmost parts of the San Benitos Islands in 1997, and it appears to be growing annually. During the breeding season of 2007, Guadalupe Fur Seals were seen on the three islands of San Benito.

Females returning to the rookery for the first time usually arrive at night or early in the morning. Estrous occurs 5-10 days after a female gives birth, and females can leave for their first foraging trip right after mating, or stay on the colony for another few days before departing. Foraging and attendance patterns are not well-known but the limited information indicates that females may travel from 700 to 4,000 km during feeding trips lasting from 4 to 24 days. Pups are weaned at 9–11 months, and females with pups can be seen on or around the island throughout the winter and into the spring.

Knowledge of activities and behaviour at sea, away from Guadalupe Island, are limited to a handful of records. At sea, they appear to be mostly solitary. In recent years they have been reported in small numbers throughout the summer in waters off Southern California and northern Baja California by long range sport fishing boats pursuing albacore. Observations of animals in captivity suggest that they spend a considerable amount of time grooming while floating at the surface. They often rest at the surface in the characteristic southern fur seal, head-down posture. They also float with one or more flippers extended out of the water. When travelling rapidly, they have been observed to “porpoise.”

Prey preference and foraging activity are poorly known. Feeding habits based on scat analysis at San Benito Islands indicated that 95% of the prey items were squid species dominated by the market squid (Loligo opalescens) which made up 65% of the identifiable prey. Stomach contents retrieved from stranded animals included a variety of squid, bony fishes, and crustaceans, including vertically-migrating species.

Killer whales and sharks, particularly Great White Sharks which are regularly seen at Guadalupe Island in the summer, are undoubtedly predators of Guadalupe Fur Seals, although there is no evidence in the literature to support this assumption. A wound on a male from a Cookie-cutter Shark bite has been reported.