It originally lived on the northeastern coast of Brazil, in the states of Piauí, Paraíba, Ceará, Rio Grande do Norte, Pernambuco, Alagoas, and Bahia.

[6] Through release (both intentional and unintentional) of captive individuals, it has expanded its range since the 1920s to Southeast Brazil (its first sighting in the wild for Rio de Janeiro was in 1929), where it became an invasive species, raising concerns about genetic pollution of similar species, such as the buffy-tufted marmoset (Callithrix aurita), and predation upon bird nestlings and eggs.

As with other members of the genus Callithrix, the common marmosets have claw-like nails known as tegulae on most of their fingers.

Other Callithrix traits shared include enlarged, chisel-shaped incisors and ceca specialized for their diet.

They have been introduced into other areas and live within the cities of Rio de Janeiro and Buenos Aires, Argentina.

[13] The common marmoset's claw-like nails, incisor shape, and gut specialization reflect their unique diet, which is primarily made of plant exudates and insects.

A marmoset may visit a tree hole multiple times, including those made by other animals.

[15] Marmosets also eat fruits, seeds, flowers, fungi, nectar, snails, lizards, tree frogs, bird eggs, nestlings, and infant mammals.

[17] Common marmosets live in stable extended families, with only a few members allowed to breed.

[18] Dominance is maintained though various behaviors, postures, and vocalizations, and subordinates groom their superiors.

Females that mate successfully but lose their young move to other groups and may gain dominant breeding positions.

[22][23] Since these suppressed individuals are likely related to the breeding pair, they have an incentive to care for the young, as they share genes with them.

The gestation period lasts for 5 months, and females are ready to breed again around 10 days after giving birth.

[17] The breeding male (likely the father) begins handling the twins, and all members of the family care for them.

At 5 months, they enter their juvenile stage, when they have more interactions with family members other than their parents, and rough play helps to establish their future status.

To signal alarm, aggression, and submission, marmosets use the "partially open mouth stare", "frown", and "slit-stare", respectively.

To display fear or submission, marmosets flatten their ear tufts close to their heads.

[28] They are used as model organisms in areas of research such as teratology, periodontal disease, reproduction, immunology, endocrinology, obesity, and aging.

[8] Segmental duplications added a total of 138 Mb of nonredundant sequences (4.7% of the whole genome), slightly fewer than observed in humans[30][31] or chimpanzees (about 5%),[32] but more than in orangutans (3.8%).