Most male harbor seals become sexually mature when they reach a weight of about 75 kg (165 lb.), at three to seven years. Most females become mature when they reach about 50 kg (110 lb.), at three to six years.

Mating season varies among the subspecies but generally occurs in late spring through fall, when females come into estrus ("heat" or "season") usually about six weeks after their pups are born. Females remain in estrus for one to nine weeks.

Harbor seals usually return to the same breeding grounds every year.

Prior to the pupping season, males and females exhibit pre-mating activity such as rolling, bubble-blowing, and mouthing each other's necks. This pre-mating behavior ends with the beginning of the pupping season.

During the mating season, male harbor seals exhibit underwater vocal displays during short dives, which are probably associated with mating. These displays take place near haul-out sites, foraging areas, and travel routes between the two areas and also increase during times that females are more likely to be in the water.

After the pupping season, males initiate true mating behavior by chasing, neck- and flipper-biting, and embracing. When approached, females respond by growling, head-thrusting, and flipper-waving. Copulation usually takes place in the water.