Mammillaria grusonii usually grows singly. This cactus is spherical to thick cylindrical, bright green, with a diameter of about. The tubercles are four-angled and contain latex. The axillae are initially woolly, later naked. The spines are straight, reddish and becomes white with age. The two central spines are 0.4 to 0.6 centimeters long, while the 12 to 14 radial spines are 0.6 to 0.8 centimeters long. The paleyellow, bell-shaped flowers are up to 2.5 centimeters long and have an equal diameter. The fruits are bright scarlet and contain brown seeds.