While not forming groups as large as some other seals, they are gregarious animals. When not actively feeding, the seals will haul themselves out of the water and onto a preferred resting site. The seals tend to hug the coast, not venturing more than 20 kilometres offshore. Both courtship and mating occurs underwater. The mating system is not known, but thought to be polygamous. Females are thought to give birth once per year, with a gestation period of eleven months.

Birthing of pups occurs annually on shore, beginning in February for populations in lower latitudes, and as late as July in the subarctic zone. The mothers are the sole providers of care with lactation lasting four to six weeks; males occupy themselves with fights between other males. Researchers have found that males gather underwater, turn on their backs, put their heads together and vocalise to attract females ready for breeding. The pups are born singly and well developed, capable of swimming and diving within hours. Suckling for three to four weeks, pups feed on the mother's rich, fatty milk and grow rapidly; born weighing up to 16 kilograms, the pups may double their weight by the time of weaning.

Common Seals must spend a great deal of time on shore when moulting (shedding off their fur), which the seals undergo shortly after breeding. This onshore time is important to the life cycle and can be disturbed when there is substantial human presence (Sullivan, 1989). A female will mate again immediately following the weaning of her pup. This pinniped is sometimes reluctant to haul out in the presence of humans, so that shoreline development and access must be carefully studied in known locations of seal haul out