Mountain Avocetbill is resident in the Andes of Colombia, Ecuador, and Peru, but is reported from relatively few localities, and its documented distribution is curiously patchy. In Colombia it is known from the west slope of the Central Andes in Caldas, on both slopes of the Central Andes in Cauca, and on the east slope of the Andes in southeastern Nariño (Meyer de Schauensee 1949, Fitzpatrick and Willard 1982, Hilty and Brown 1986). In Ecuador the avocetbill is reported from sites along the length of the east slope of the Andes, south to Loja and Zamora-Chinchipe (Ridgely 1980, Ridgely and Greenfield 2001a). This species is not (yet?) reported from Peru north of the Río Marañón, but is reported from two sites on the east slope of the Andes south of the Marañón, in Amazonas and in eastern La Libertad (Ridgely 1980, Fjeldså and Krabbe 1990, Schulenberg et al. 2010).
The elevational range of Mountain Avocetbill in Colombia is 2600-3600 m (Hilty and Brown 1986); mostly 2400-3200 m in Ecuador, with one record as low as 1700 m (Ridgely and Greenfield 2001a); and 2700-3200 m in Peru (Schulenberg et al. 2010).