Head and body length of S. carolinensus is between 200-315 mm, tail length is 200-310 mm. Weights range from 200-1,000 grams. Upperparts are grey to greyish brown with paler, sometimes whitish, underparts. There are two molts per year, but the tail fur is only shed once each year.

S. carolinensus inhabits deciduous, coniferous and tropical forests, both humid and arid. Most of their time is spent in the treetops, but they do come down to forage for food and bury nuts and acorns. Nests are constructed in hollow tree cavities or in tree branches during the summer. While S. carolinensus may have a particular den that is used for several years, others are constructed as shelters used to escape from enemies. S. carolinensus is primarily diurnal, but in the northern part of its region, will forage midday during the winter months. While Grey squirrels do not hibernate, they will remain in their nests during very cold or stormy weather, unless it is necessary for them to find food. S. carolinensus will migrate if food stores become low. Mass migrations, which can involve thousands of animals, occur when the food supplies are low and population densities are high.

The diet of S. carolinensus consists of various nuts, seeds, fruits, buds and young tree shoots. Acorns and hickory nuts are favorites when available.

The gestation period for S. carolinensus is about 44 days, with an average litter size of 3, but as many as 9 young have been recorded in a single litter. Weighing less than an ounce, the babies are born naked and blind. Fur begins to grow at about 14 days and the eyes open around 30-32 days. They remain in the nest for approximately six weeks. Females generally breed within their first year.

S. carolinensus is found in E Texas (USA) to Saskatchewan (Canada) and east to the Atlantic coast. Introduced into Britain, Scotland, Ireland, Italy, Australia, South Africa and various localities in W North America.