The first species was described by Carl Linnaeus as Cactus mammillaris in 1753, deriving its name from the Latin mammilla, "nipple", referring to the tubercles that are among the distinctive features of the genus.

The distinctive feature of the genus is the possession of an areole split into two clearly separated parts, one occurring at the apex of the tubercle, the other at its base.

The plants are usually small, globose to elongated, the stems from 1 to 20 centimetres (1⁄2 to 7+3⁄4 inches) in diameter and from 1 to 40 cm (1⁄2 to 15+3⁄4 in) tall, clearly tuberculate, solitary to clumping forming mounds of up to 100 heads and with radial symmetry.

The flowers are funnel-shaped and range from 7 to 40 millimetres (1⁄4 to 1+1⁄2 in) and more in length and in diameter, from white and greenish to yellow, pink and red in colour, often with a darker mid-stripe; the reddish hues are due to betalain pigments as usual for Caryophyllales.

[1] Mammillaria is a large and diverse genus with many species often exhibiting variations due to the nature of terrain, weather, soil and other ecological factors.

[5] Over time, new investigators began grouping closely related forms under the same name to attempt to more accurately define the species.

Intense studies of DNA of the genus are being conducted, with preliminary results published for over a hundred taxa, and this promising approach might soon end the arguments.

As of November 2024[update], Plants of the World Online accepted the following species:[6] Mammillarias is predominantly found in Mexico but also have a wide range of distribution in neighboring regions north of the equator including the southwest United States, the Caribbean, Guatemala and Honduras.

Cactus fanciers can assist conservation of these rare plants by choosing nursery-bred specimens (wild-collected ones are illegal to possess for the rarest species).