Northwest Atlantic harp seals are migratory, generally spending the summer months in arctic and subarctic waters between Greenland and the eastern Canadian Arctic and the winter months in the waters off the east coast of Canada. They begin the trip south in late September, appearing off the coast of Labrador and in the Gulf of St. Lawrence in late December and early January.

In February and March, harp seals congregate in extensive patches to give birth to their pups on the ice off of Newfoundland (the "Front") and in the Gulf of St. Lawrence (the "Gulf"). These congregations are known as whelping patches.  Once the females are free from nursing their pups, they are courted by adult males. Mating generally occurs in the water, but has also been observed on the ice. After mating, the seals move to ice farther north to undergo the annual moult, shedding their old hair coats for new ones. After the moult, the seals continue northward to summer feeding grounds between northern Canada and the west coast of Greenland, and the annual cycle begins once more.

Throughout the 1990s, there have been increased sightings and strandings of harp seals in the southern reaches of their range, from the Gulf of Maine to New Jersey. Several suggestions have been made to explain this recent change in distribution. With the depletion of several commercial fish stocks off the east coast of Canada in the late 1980s and early 1990s, perhaps the seals are moving southward in search of food. Alternatively, changing climatic conditions may be affecting the distribution of prey species, and the seals may simply be following their prey as they move southward with the advancing cold northern waters (McAlpine et al. 1999).