For healthy growth, make sure that Mammillaria mystax receives at least 3–6 hours of sun each day. This is actually a minimum requirement—most plants that can handle part sun can also thrive in full sun, but because they require less light for photosynthesis, they are more flexible than plants that require full sun or part shade.

Mammillaria mystax does best with exposure to full or part sun. They will perform best with direct morning light, but in summer they need protection from the strong afternoon sun. In temperate environments, too much hot afternoon sun can burn the leaves, damaging the plant's appearance and health.

Mammillaria mystax planted indoors can easily be damaged by direct sunlight when it's moved outdoors. The best way to prevent sunburns from overexposure is to move pots gradually from a shaded area to a brighter spot, gradually. But even plants that are acclimated to the summer sun can be damaged by extreme heat. In a heatwave, it is important to keep the soil consistently moist so that plants can cope with excessive levels of heat. Moving plants in containers to areas with afternoon shade or erecting a shade cloth over them can protect sensitive Mammillaria mystax during extreme weather events.

While bright morning sun and some full sun exposure can be highly beneficial for Mammillaria mystax, the harsh, hot midday sun of summer can be too much to handle. If planted in the ground, the summer sun will usually ramp up slowly enough through the season for Mammillaria mystax to gradually adapt to its intensity. But a potted plant that has been indoors or in a protected location will often suffer injury when placed suddenly into a location where the direct summer sun reaches it in the hottest part of the day. To protect this plant from the brutal afternoon summer sun, plant or place it in an understory location where it is shaded at midday by taller trees and plants or by a building or landscape feature.

When Mammillaria mystax receives too little sun, they may become pale green or display drooping, yellow leaves. While some leaf drop is normal, if leaves are dropping but no new ones are growing in to replace them, it is a sign that something is wrong. If Mammillaria mystax receiving inadequate light does manage to grow, the new growth is often spindly, pale, and prone to insect infestation. Paying attention to these signs and changing the lighting conditions of the plant will make a significant difference.

Tender, new leaves are especially sensitive to sunburn. Bearing this in mind, very young Mammillaria mystax and when it's in a strong growth phase, such as in late spring and early summer, will be more sensitive to harsh sun and heat than the mature one or those in a more dormant fall growth stage. Mammillaria mystax fresh from a nursery is also usually not prepared for strong full sunlight and must be introduced to it slowly.

Recently transplanted Mammillaria mystax will often experience a bit of shock and will need to be cared for carefully, either shaded from bright afternoon sun or placed in a protected area. On very hot days, you may see the leaves of Mammillaria mystax drooping—this is usually nothing to worry about. Plants will send the water in their leaves down into their roots to protect them from burning. However, if the leaves are still drooping in the evening or the next morning, the plant needs water. Always avoid watering during the hottest times of day, as sunlight can hit wet leaves and scorch them easily. Mammillaria mystax that has been underwatered will be weaker than that with consistently moist soil. This can leave it with weak roots that are unable to protect the leaves on hot, sunny summer days by diverting water away from the leaves. Care for an underwatered plant by giving it a long, deep watering and then allowing the top two inches of soil to dry out before the next watering. Even if it loses its leaves, if cared for properly it will grow new ones.