Black-legged Dacnis is endemic to and found sporadically throughout the coastal region of southeast Brazil; the distribution of this species in part defines the Serra do Mar area of endemism (Silva et al. 2004). Within this region, however, it is distributed somewhat patchily, from central Espírito Santo south through Rio de Janeiro and São Paulo to eastern Paraná and northeastern Santa Catarina (including Santa Catarina Island) (Storer 1970, Piacentini et al. 2006, Hilty 2011, Lombardi et al. 2012). There also is a 19th century report from south central Minas Gerais (Burmeister 1856), but it is suspected that the locality data for this specimen may be incorrect (Isler and Isler 1987, Collar et al. 1992). Collar et al. (1992) suspected, however, that Black-legged Dacnis might occurs in extreme southeastern Minas Gerais, a prediction that appears borne out by a report from extreme southern Minas (Lombardi et al. 2012).
The center of elevational abundance for Black-legged Dacnis is in the hill tropical zone (Parker et al. 1996); it is mostly found in the lowlands, commonly ranging from ca 470 m to 850 m (Cordeiro 2001, Ridgely and Tudor 2009, van Perlo 2009). Seasonally, and in low numbers, it can be found in elevations up to 1700 m (Parker et al. 1996, van Perlo 2009, Hilty 2011). Black-legged Dacnis apparently is at least partially an altitudinal migrant, as it occurs in the lowlands in Rio de Janeiro only during the austral winter (Sick 1993, Pimentel and Olmos 2011). It also apparently is somewhat nomadic, appearing in small groups in areas where favorable plants are abundant, but with patterns of occurrence that are variable from year to year (Collar et al. 1992, Willis and Schuchmann 1993, Whittaker et al. 2010, Hilty 2011). It was recorded fairly regularly in point counts in a restinga forest of São Paulo (Dario and de Vincenco 2011), although these authors did not discuss seasonal patterns of occurrence.
This species occurs in the Atlantic Forest Zoogeographic Region, in the Southern Atlantic Coast subregion (Parker et al. 1996).