It is native to North America west of the Rocky Mountains (California to British Columbia, east to Idaho Minnesota and Nevada) but has in recent times expanded its range to eastern North America, to include Ontario, Québec, New Brunswick, Nova Scotia, Michigan, Maine, Pennsylvania, New York, Connecticut, Massachusetts, New Hampshire, Ohio, Indiana, and Wisconsin, and has become an accidental introduced species in parts of Europe and Argentina.

These, being Cimicomorpha, are not very closely related to leaf-footed bugs as Heteroptera go; though both have a proboscis, but only the assassin bugs bite even if unprovoked, and L. occidentalis like its closest relatives can be most easily recognized by the expanded hindleg tibiae and by the alternating light and dark bands which run along the outer wing edges on the flaring sides of the abdomen.

Their primary defense is to emit an unpleasant-smelling alarm pheromone;[2] however, if handled roughly they will stab with their proboscis, though they are hardly able to cause injury to humans as it is adapted only to suck plant sap and not, as in the assassin bugs, to inject venom.

In the northern parts of its range, these bugs start to move about widely by September or so to seek crevices for overwintering; they may become a nuisance in areas with extensive conifer woods, as they will sometimes enter houses in considerable numbers.

[4] This insect is common in its native range along the temperate and warmer regions of the Pacific coast of North America and has steadily expanded eastwards.

By 2007, it had established itself in the northern Balkans (Slovenia, Bosnia and Herzegovina and Croatia), the Alps (Austria, Switzerland), and parts of the Czech Republic, France, Germany and Hungary; in 2003, it was found to occur in Spain, though this population probably derives from a separate introduction.

[3] During the autumn of 2008, a large influx of this species arrived on the south coast of England, indicating natural immigration from continental Europe.