Adult black-capped vireos weigh 9 to 10 grams (0.32 tp 0.35 ounces).

One of the smallest in the vireo family, the black-capped vireo measures 11 to 12 centimeters (4.3 inches to 4.7 inches) in length. 

Male and female black-capped vireo have slightly different coloration as adults. Males are olive-green on the back, mostly white below with flanks of faint greenish-yellow. The species gets its name from its mostly black head or cap and prominent white spectacles extending to its black bill. Mature female coloration is slightly duller than males, with the cap on the head a medium to dark gray.

Male black-capped vireos sing an extensive song made up of distinct phrases of 2 to 4 notes to defend territories. Females do not sing, but do vocalize through calls, as do males, to signal to their mate, as an alarm or to scold potential predators.

Following a short courtship, black-capped vireos will begin constructing a nest within their habitat. The nest is usually constructed by both the adults in a shrub with leafy foliage extending to the ground, which helps to conceal the nest from predators. The female will lay three to four eggs, which both parents will incubate. Eggs hatch after 15 to 16 days, and chicks are born naked and blind. When 11 or 12 days old, young vireos begin to leave the nest, or fledge, with both adults tending and feeding them until they are independent. This usually takes between 35 to 45 days after fledging.

Black-capped vireos migrate from their wintering grounds in Mexico and arrive in Texas and Oklahoma from mid-March to mid-April. Males generally arrive one to two weeks before females and establish territories, ranging from 1 to 1.9 hectares (2.5 to 4.6 acres) in size, by singing along the boundaries. Males and females pair immediately upon arrival and select nest sites together. Although not common, adult female vireos may successfully rear two broods in a single season. Fall migration begins in July, but most occurs in August in September. The adults spend their winter in western Mexico, to return to the breeding grounds the following spring.

Black-capped vireos live several years, with most individuals living between two and three years. The longest documented lifespan of an individual is 12 years.

Black-capped vireo habitat varies by the seasonal needs of their winter, or non-breeding, and spring/summer, or breeding, ranges. On the wintering grounds of western Mexico, habitat is known to have a wide range of vegetation types described as arid scrub and a more diverse secondary growth and forest edge. Breeding and nesting habitat is generally described as shrublands and open woodlands. They use low scrubby growth, which is generally under 10 feet in height, of mostly deciduous shrubs and trees of irregular height and distribution. These habitats are characterized as having foliage cover to ground level, with spaces between shrub/tree mottes, or groups of woody plants. Woody shrub/tree mottes cover 35% or greater of the area, while 35 to 55% is preferred. The open spaces between mottes consists of bare ground, rock, grasses and forbs. In the eastern portion of the breeding range, habitat is often described as early successional or dependent on periodic disturbance from activities like wild or prescribed fire to maintain suitable conditions. In Mexico, breeding habitat is similar to that of the United States, but often lacks any open space between woody shrubs. 

Adult and fledgling black-capped vireos forage for insects and spiders mostly by gleaning them from vegetation. While their diet is mostly arthropods, especially butterfly and moth larvae, they will also supplement with seeds and other plant matter. Nestlings are fed mostly small insect larvae by both adult parents.

Male black-capped vireos sing daily during the entire breeding season, although the frequency diminishes in July and August. Singing establishes a territory boundary and attracts a female for courtship. Black-capped vireo territories are often in clusters or groupings within a habitat patch. Adults have a strong site fidelity between breeding seasons, often returning to the same territory or habitat patch each year. First year breeding adults, hatched from the previous breeding season, usually establish territories further from their hatch site.