Adults weigh from 23.2 to 56.5 g (0.8 to 2.0 oz), with an average of 35–40 g (1.2–1.4 oz) They range in length from 20.5 to 24 cm (8.1 to 9.4 in) and span 22 to 30 cm (8.7 to 11.8 in) across the wings. Among standard measurements, the wing chord is 8.4 to 9.8 cm (3.3 to 3.9 in), the tail is 7.2 to 10.3 cm (2.8 to 4.1 in), the culmen is 1.5 to 1.8 cm (0.6 to 0.7 in) and the tarsus is 2.7 to 2.9 cm (1.1 to 1.1 in). Gray catbirds are plain lead gray almost all over. The top of the head is darker. The undertail coverts are rust-colored, and the remiges and rectrices are black, some with white borders. The slim bill, the eyes, and the legs and feet are also blackish. Males and females cannot be distinguished by their looks; different behaviours in the breeding season is usually the only clue to the observer. Juveniles are even plainer in coloration, with buffy undertail coverts.
This species is named for its cat-like call. Like many members of the Mimidae (in particular mockingbirds), it also mimics the songs of other birds, as well as those of Hylidae (tree frogs), and even mechanical sounds. Because of its well-developed songbird syrinx, it is able to make two sounds at the same time. The alarm call resembles the quiet calls of a male mallard.

A gray catbird's song is easily distinguished from that of the northern mockingbird (Mimus polyglottos) or brown thrasher (Toxostoma rufum) because the mockingbird repeats its phrases or "strophes" three to four times, the thrasher usually twice, but the catbird sings most phrases only once. The catbird's song is usually described as more raspy and less musical than that of a mockingbird.

In contrast to the many songbirds that choose a prominent perch from which to sing, the catbird often elects to sing from inside a bush or small tree, where it is obscured from view by the foliage.

Native to most of temperate North America east of the Rocky Mountains, gray catbirds migrate to the southeastern United States, Mexico, Central America, and the Caribbean in winter; except for the occasional vagrant they always stay east of the American Cordillera. They are extremely rare vagrants to western Europe. Normally present on the breeding grounds by May, most leave for winter quarters in September/October; as it seems, this species is increasingly extending its stay in the summer range, with some nowadays remaining until mid-winter as far north as Ohio. The gray catbird is a migratory species. Spring migration ranges from March to May, and in the fall ranges from late August to November.

The catbird tends to avoid dense, unbroken woodlands, and does not inhabit coniferous, pine woodland. Catbirds prefer a dense vegetative substrate, especially if thorny vegetation is present. Scrublands, woodland edges, overgrown farmland and abandoned orchards are generally among the preferred locations of the catbird. In Bermuda, its preferred habitats are scrub and myrtle swamp. During the winter season, the catbird has an affinity for berry-rich thickets, especially within proximity of water sources.

Their breeding habitat is semi-open areas with dense, low growth; they are also found in urban, suburban, and rural habitats. In the winter months they seem to associate with humans even more. These birds mainly forage on the ground in leaf litter, but also in shrubs and trees. They mainly eat arthropods and berries. In the winter months, Cymbopetalum mayanum (Annonaceae) and Trophis racemosa (Moraceae) bear fruit well liked by this species, and such trees can be planted to attract the gray catbird into parks and gardens.

They build a bulky cup nest in a shrub or tree, close to the ground. Eggs are light blue in color, and clutch size ranges from 1–5, with 2–3 eggs most common. Both parents take turns feeding the young birds.
Gray catbirds are omnivores, and approximately 50% of their diet is fruit and berries. They tend to peck more fruit than they can eat. They also eat mealworms, earthworms, beetles, and other bugs. In summer, gray catbirds will eat mostly ants, beetles, grasshoppers, caterpillars, and moths. They also eat holly berries, cherries, elderberries, poison ivy, bay, and blackberries. They also often peck the eggs of other species of birds, but it is unknown if they do this to supplement their diet or to reduce competition for food from other birds.