Described by Oberholser in 1918, it breeds in California from Tulare County north along the western side of the Sierra Nevada, and in Oregon and Washington west of the Cascade range.

Populations at high elevation just east of the Sierra Nevada crest but south of Modoc County are assumed to be E. t. brewsteri.

The southwestern willow flycatcher (E. t. extimus) is a federally endangered subspecies found in Arizona, California, Colorado, Nevada, New Mexico, Texas, and Utah.

In 1996, breeding was confirmed along the Arizona side of the lower Colorado River at Lake Mead Delta and at Topock Marsh.

[5] In two sites, one in Arizona and the other in New Mexico, native trees were able to replace patches of saltcedar and populations of willow flycatchers increased.

[8] North American beavers (Castor canadensis) are thought to play a critical role in widening riparian width, openings in dense vegetation, and retention of surface water through the willow flycatcher breeding season.

Their breeding habitat is deciduous thickets, especially willows and often near water, across the United States and southern Canada.

Willow flycatchers travel approximately 1,500–8,000 km (930–4,970 mi) each way between wintering and breeding areas.

Willow flycatchers are generalist insectivores, and the insects which comprise their diet vary substantially across different habitats.