Its most distinctive characteristic is the long pectoral fins that reach nearly to the base of the pelvic fins, and thus inspire the common name. The sides are silver, with the back ranging from an olive to a pinkish shade. The upper jaw is long, reaching nearly to the posterior edge of the eye, and the lower jaw projects slightly in front of the upper jaw. Both jaws have set of very small teeth. The lateral line is incomplete, reaching back only as far as the dorsal fin. Size is limited to about 20 cm.
Their principal food item is the opossum shrimp, Neomysis mercedis, and species of Acanthomysis, but they will also eat copepods and other small crustaceans. In turn, they are eaten by a variety of fishes, birds, and marine mammals; for instance, they are an important prey for the harbor seal, Phoca vitulina, in the Columbia River.
Their primary habitat is the open water of estuaries, where they can be found in both the seawater and freshwater areas, typically in the middle or deeper parts of the water column. They have been found as far north as Prince William Sound in Alaska, also in Skagit Bay, Grays Harbor, Willapa Bay, lower Columbia River, Yaquina Bay, Coos Bay, Humboldt Bay, the Eel River estuary, Klamath River estuary, and San Francisco Bay. They have been collected from the mouth of the Russian River occasionally, and a single fish was once caught in Monterey Bay. In addition, there are landlocked populations in British Columbia's Harrison Lake, and the Lake Washington.
Although once one of the most common species found in the San Francisco and Humboldt bays, even as late as the 1970s, they are now much less frequent in the smelt fishery. In 1992 the Natural Heritage Institute petitioned to list longfin smelts as an endangered species, but the petition was denied the following year, among the reasons being given was that the decline was not observed elsewhere. The reasons for decline are not known; Moyle suggests estuary outflow reduction, entrainment is connection with the pumping of water out of the Delta area, climatic variations, water pollution, and the impact of introduced species as possibilities.