Plain-capped Starthroat occurs from northwestern Mexico south along the Pacific slope to northwestern Costa Rica (Griscom 1932, Dickey and van Rossem 1938, Friedmann et al. 1950, Stiles and Skutch 1989, Howell and Webb 1995). It also occurs locally in the interior, such as in the Balsas drainage in southern Mexico (Friedmann et al. 1950, Howell and Webb 1995), in Honduras (Monroe 1968), and in Costa Rica (Stiles and Skutch 1989).
In Mexico it occurs north to southeastern Sonora. Van Rossem (1945) and Russell and Monson (1998) suggest that Plain-capped Starthroat is only a summer resident in Sonora, where it primarily is present from mid-March to August (Russell and Monson 1998). Nonetheless van Rossem (1945) mentioned a February record from Sonora, and eBird data suggest that it may be present there year round, although in smaller numbers during the winter.
Plain-capped Starthroat also is a rare nonbreeding visitor to Arizona, southwestern United States, where it first was reported in 1969, and more regularly and the late 1970s (Witzeman 1979); it primarily occurs from May through October (eBird data).
The elevational range of Plain-capped Starthroat in Mexico is from sea level up to 1500 m (Howell and Webb 1995), but only to 1000 m in Costa Rica (Stiles and Skutch 1989).