Sialia mexicana, commonly known as the Western Bluebird, is native to North America. Its geographical distribution includes regions in the western United States, extending from the Pacific coast, including states like California and Washington, and stretches inland to western Texas. The species is also found in parts of Mexico, including the Baja California Peninsula, and occasionally in southern British Columbia, Canada.

Western Bluebirds inhabit open woodlands, forest edges, and scrublands. They prefer environments with scattered trees and a combination of open grassy areas or meadows that provide adequate insect populations for foraging. The climate within these habitats can range from temperate to semi-arid, depending on the specific location within their range.

The ecological niche of the Western Bluebird involves feeding on a mixed diet of insects and berries, which makes them important for controlling insect populations. They are cavity nesters, often relying on natural tree cavities or old woodpecker holes, and will also use nest boxes if available. Variations in habitat do occur, particularly for populations in more arid environments, where they may be adapted to rely more heavily on available water sources and desert vegetation.