Cassin's auklet ranges from midway up the Baja California peninsula to Alaska's Aleutian Islands, off North America.

Two subspecies are recognised:[2] An extinct relative or predecessor species, Ptychoramphus tenuis L. H. Miller et Bowman, 1958, is known from the Late Pliocene of the San Diego Formation in California.

Cassin's auklet nests in burrows on small islands, and in the southern area of its range may be found in the breeding colony year round.

Both the parents incubate the single white egg, returning to swap shifts at night (usually after 2300 h) to avoid being taken by predators such as the western gull or peregrine falcon.

[3] At sea Cassin's auklets feeds offshore, in clear often pelagic water, often associating with bathymetric landmarks such as underwater canyons and upwellings.

Threats to the auklet include introduced carnivores (particularly in Alaska), oil spills and changes in sea surface temperature (caused by El Niño events).

[6] In the last few months of 2014, the carcasses of thousands of Cassin's auklets washed ashore from Northern California up to the north coast of Washington State.

Some scientists from the California's Farallon Institute believe these mass deaths could be related to the unusual North Pacific warmth which is pushing marine food chains[8][9] and could affect other species of zooplankton, krill and fish that normally develop in cold waters and the birds that consume them including Cassin's auklets.