Lyall's mariposa lily only occurs in British Columbia and Washington state. On the Canadian side of the border, it is considered a threatened plant (COSEWIC report on Calochortus lyallii in PDF format), as it can be found in only a few locations within 5km (3 miles) of the Canada-US border. In Washington state, it is relatively more common in the northern and central interior regions, where it occasionally occurs in large, dense populations (distribution map).

A short (to 20cm or 8in) plant of dry, open Douglas-fir and pine forests, it is threatened by cattle grazing and inappropriate reforestation practices post-burn or post-logging (too dense of a canopy will reduce its population numbers significantly). Competition from weedy invaders can also be a problem. The story of cattle grazing and weeds being problematic for native plants is repeated ad infinitum throughout the intermountain area, we observed.

Of the 56 species of Calochortus that occur in North America north of Mexico, we encountered 5 species during our expedition. That number may not seem like a lot, but most of those 56 species are California endemics (where we didn't visit). For those species that do occur outside of California, I think we encountered nearly half of them.