Mammillaria magnimamma is a perennial globose plant reaching a height of 15–30 cm and a diameter of about 13 cm. At first it grows solitary, but later forms large clumps rising above ground level. Tubercules are four-sided, with latex and the axils have dense white wool. The radial spines are 2 - 5, quite variable and unequal, with dark tips, 15 – 45 mm long. Flowers are pink or white cream with reddish midveins and a diameter of 20 – 25 mm. They bloom in mid Spring. Fruits are club shaped, dark red, about 20 mm long, and contain little brown seeds.