Guadalupe Fur Seals are sexually dimorphic, with territorial males being 3.8 to 4 times larger than adult females (Gallo-Reynoso and Figueroa-Carranza 1996). Fourteen adult females averaged 148.2 cm standard length (tip of nose to tip of tail) and their weight averaged 49.1 kg. Twelve territorial males averaged 219 cm (Gallo-Reynoso and Figueroa-Carranza 1996), and males averaging 180 cm weighed 160-170 kg (Bonner 1994).

Guadalupe Fur Seals are polygynous, with males establishing territories that are occupied by a small number of females (Peterson et al. 1968). Pups are born from mid-June to August with a median birth date of 21 June (Wickens and York 1997). From 1991 to 1993 the median peak pupping date was 3 July at Guadalupe Islands (Gallo-Reynoso 1994). For up to 31 days, males defend their territory with vocalizations, displays, and mutual displays with neighbouring bulls (Gallo-Reynoso 1994). Guadalupe Fur Seals prefer shores with abundant large rocks and lava blocks, often at the base of large cliffs.They inhabit caves and recesses, which provide protection and cooler temperatures, especially during the warm breeding season. This segregates them from other pinniped species (Peterson et al. 1968, Pierson 1987, García-Aguilar et al. 2013).

Oestrus occurs 5-10 days after a female gives birth, and females can leave for their first foraging trip right after mating, or stay at the colony for a few days before departing (Peterson et al. 1968, Pierson 1987, Gallo-Reynoso 1994). Foraging and attendance patterns are not well-known but the limited information from four instrumented adult females indicates they travel a total mean distance of 2,375 km. The average distance to the feeding grounds was 444 km and the duration of feeding trips averaged 14.4 days. Diving records for one animal showed a deepest dive of 82 m (Gallo-Reynoso et al. 2008).

Feeding habits are poorly known, particularly for the main colony on Guadalupe Island where the few scats samples analysed to date included remains of the cephalopods Onychoteutis banksi, Eucleoteuthis luminosa, and Dosidicus gigas, as well as several fish, including Scomber japonicus, Auxis thazard, and Sardinops sagax (Gallo-Reynoso 1994). A study conducted during 2000-2001 on the San Benito Islands examined 218 Guadalupe Fur Seal scat samples in which 95.6% of the prey were cephalopod beaks including mostly Loligo opalescens and in lesser abundance Gonatus sp. and Dosidicus gigas. The few fish remains (4.4%) identified were mostly from Merluccius productus, Engraulis mordax and Sardinops sagax (Aurioles-Gamboa and Camacho-Ríos 2007). A similar diet was described in another study conducted during the summer of 2007 (50 scats) and winter of 2008 (56 scats) with the squid Loligo opalescens and Gonatus sp., making up 74% of the summer diet. In winter, these two squid species, along with Dosidicus gigas composed 87% of the prey. The fish Argentina sialis, Merluccius productus, and Sebastes spp. complemented the Guadalupe Fur Seal diet in both seasons (Pablo-Rodríguez 2009). Esperón-Rodríguez and Gallo-Reynoso (2013) and Gallo-Reynoso and Esperón-Rodríguez (2013) also found Loligo opalescens as the main summer prey for juveniles and subadult males at San Benito Islands. 

Contents of faeces and gastrointestinal tracts from several individuals stranded off the coast of northern California near Farallon Island included Loligo opalescens, Gonatopsis sp., Onychoteuthis borealis japonica, as well as several fish species: Citharichthys sordidus, Lampanyctus sp., Protomyctophum sp., and Scopelogadus sp. (Hanni et al. 1997).

Killer Whales and Sharks, particularly Great White Sharks, are regularly seen around Guadalupe Island during the summer, and are most likely predators of Guadalupe Fur Seals. These two predators are rarely seen around the San Benito Islands.

The distribution of the Guadalupe Fur Seal is centred on Guadalupe Island (Fleischer 1987, Gallo-Reynoso 1994, Belcher and Lee 2002, Aurioles-Gamboa et al. 2010). A small breeding colony was discovered on the east side of the easternmost part of the San Benito Islands in 1997 (Maravilla and Lowry 1999); now animals are present and spreading on all three San Benito Islands (Aurioles-Gamboa et al. 2010, García-Capitanachi 2011, Esperón-Rodríguez and Gallo-Reynoso 2012). Since the 1980s, Guadalupe Fur Seals have been observed more frequently on San Miguel Island and other southern California islands including, the Farallon Islands off the coast of northern California (Bartholomew 1950, Stewart 1981, Stewart et al. 1987, Stewart et al. 1992, Hanni et al. 1997). Guadalupe Fur Seal vagrants have been sighted and recorded stranded in California, Oregon, and Washington, USA, particularly during El Niño years (Hanni et al. 1997, Etnier 2002). Since 2009, around 50 Guadalupe Fur Seals have been found on the beaches along the west coast of Magdalena Island, located around 25ºN, approximately 500 km south of the San Benito Islands (Aurioles, unpublished data). The species' distribution at sea is poorly known, but records from a few satellite-tracked adult females suggest they may travel several hundred kilometres during feeding trips (Gallo-Reynoso 1994). Guadalupe Fur Seals have been sighted regularly in the Gulf of California, Mexico (Aurioles-Gamboa et al. 1999) at the southern extent of their distribution.

Guadalupe Fur Seals were the most frequently encountered pinniped in archaeological deposits on the San Miguel Island prior to the seal exploitation period (Walker and Craig 1979). Indeed, its abundance in deposits dating from 3,500 years to the present, between 32°N and 50°N latitude, suggests that the highest density of sites and individuals occurred on the Channel Islands and southern parts of the mainland (34-36°N), with densities declining north of Point Conception (Rick et al. 2009).

The Guadalupe Fur Seal and its habitat are protected by the Mexican government and tourist visits to breeding islands are regulated and very limited. In the United States, the species is protected by the Marine Mammal Protection Act and the Fish and Game Code of the State of California. The Guadalupe Fur Seal was listed as threatened under the U.S. Endangered Species Act in 1985, which automatically assigned the species the status of depleted under the Marine Mammal Protection Act.