As its name suggests, the Common Hippopotamus is an amphibious creature, which spends the day in water and emerges at night to feed. The Hippopotamus uses the water only as a retreat and typically does not eat aquatic vegetation, rather they forage on grass at varying distances from a water source. Hippos rely on a range of water sources – rivers, lakes, and wetlands although seasonally the animal can survive in muddy wallows.  Hippos require some form of permanent water particularly in the dry season as their skin must remain moist and will crack if exposed to the air for long periods. The skin physiology is complex and not fully understood but is clearly adapted for an amphibious existence. Hippos secrete a red liquid from subdermal glands which is thought to function as both a sunscreen and antibiotic (Saikawa et al. 2004).

Common Hippos are gregarious, social, polygymous animals when resting in water by day, with herd sizes ranging from the tens to a hundred. The social habits of the species have been studied by Klingel (1991), who found that the "schools" are groups of females and juveniles. The social system is based on mating territoriality. Females become sexually mature between the ages of 7–9, and males 9–11. Females typically bear a single offspring every other year as lactation can extend for 18 months or longer. In captivity, reproductive activity and sexual maturity occurs in younger individuals. Territorial males monopolize a length of the shoreline of the river or lake but tolerate bachelors within the territory provided they behave submissively. Non-breeding males also settle outside territorial areas, especially seasonal wallows. Fights for the possession of a territory can be fierce and the animals may inflict considerable damage on each other with their huge canines but minor conflicts are usually settled by threat displays, of which the "yawn" is the most conspicuous. Territorial males do not normally fight each other and severe fights usually occur only when a bachelor challenges a territorial male for control of its territory. There is little association between animals when they are feeding at night, except between females and their dependent young, and the males do not then behave in a territorial fashion.

The male Common Hippo, rarely the female, spreads its dung by wagging its tail vigorously while defecating, both in the water and on land, where it is thought to have a signalling rather than a territorial function. The dung piles may serve to orient Hippos or may mark territories.

Vocalizations take the form of complex bellows and grunts, which presumably have a signalling function. Sounds may be made either on land or in the water and may be transmitted simultaneously through air and water. This is the only known case of amphibious calls in a mammal.

It is likely that thermoregulation has determined the nocturnal feeding habits of the animal. Hippos typically leave wallow or pool soon after sunset and spend night hours grazing on short grass swards for up to several kilometres from water. The distance animals travel to grazing areas likely varies seasonally and among different areas. Grazing areas, which are kept short by the activities of the Hippopotamus, are often referred to as Hippo lawns. Although Hippos can graze every night, there are usually animals present in the water all night, as some return after a few hours and others leave later. Mothers with very young calves represent the only apparent social structure at grazing areas  as there is no clear organization at the grazing lawn.The animal feeds by plucking the grass with its wide, muscular lips and passing it to the back of the mouth to be ground up by the molars. The front teeth (incisors and canines) play no part in feeding. The amount of food ingested is small relative to the size of the animal but its resting habits by day reduce its energetic demands and food may be retained in the gut longer than most large herbivores. The stomach is a complex four-chambered structure with ruminant-type digestion although Hippos are not true ruminants and do not chew the cud.The ecological requirements for Hippopotamus, therefore, include a supply of permanent or seasonal water, and adequate grazing on open grassland within a few kilometres of the aquatic habitat.

Common Hippopotamus (referred to as Common Hippos or Hippos hereafter) are found in many countries throughout sub-Saharan Africa in suitable wetland habitats. The species still occupies much of its former range from 1959, although population sizes have declined. Common Hippos occur in rivers throughout the savanna zone of Africa, and main rivers of forest zone in Central Africa, in Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Côte d’Ivoire, Democratic Republic of Congo, Ethiopia, Equatorial Guinea, Gabon, Gambia, Ghana, Guinea, Guinea Bissau, Kenya, Rwanda, Senegal, Sierra Leone, Somalia, Sudan, South Sudan, Swaziland, Malawi, Mozambique, Namibia, Niger, Nigeria, Republic of Congo, Sierra Leone, South Africa, Tanzania, Togo, Uganda, Zambia, and Zimbabwe.

Common Hippos were already rare in Egypt by the time of the Renaissance. From the end of the Roman Empire up until towards 1700 at the latest, the Hippo was still present in two well disjunct zones in the Nile Delta and in the upper Nile. Through the 1700s, records become increasingly scarce, and the latest definite records are from the early 1800s (Manlius 2000).

This species is listed on CITES Appendix II.
There are numerous protected areas across the countries where Common Hippos are found. Although in most countries the official level of protection is good, the level of enforcement of these regulations is poor in many countries. In some countries, Common Hippos are still found outside of protected areas.