M. trossulus is found in the mid intertidal to subtidal zone. It prefers protected waters and beaches where it often forms large beds. This species has a widespread but broken distribution. This may be because present populations are remnants of a previously more widespread species that was disconnected during the Pleistocene ice ages.  Populations are currently found from the Arctic Ocean to northern California, eastern Canada, Baltic Sea and eastern Russia.

A dense community of M. trossulus and other intertidal animals on False Outer Point beach Juneau, Alaska.

In its intertidal home, M. trossulus plays several important ecological roles. M. trossulus is an important food resource in the intertidal. Numerous predators such as echinoderms (ex Leptasterias hexactis, Pisaster ochracheus, and Evasterias troschelii), anemones, whelks (ex. Nucella sp.), crabs, fish, ducks and shorebirds (Larus sp. and Aphriza virgata) eat M. trossulus. Many of these predators act to set a lower limit for the mussel. M. trossulus is also important ecologically as an intertidal sanctuary for numerous animals which live beneath and amongst them. Members of this assemblage include gammarids, isopods, mollusks, polychaetes, and nemerteans. Their digestive tract is home to a copepod parasite (Mytilicola orientalis). Finally, as a filter feeder, M. trossulus helps clear the water column by consuming mainly detritus as well as some planktonic organisms. It filters an amazing 2.49 liters of water per hour per gram of body weight. The filibranch type gills are capable of removing particles as fine as 4-5 µm from the water. The water that flows out of the excurrent siphon carries less debris than what entered the mussel.

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Leah Behrends, 2002.