A mammoth is any species of the extinct elephantid genus Mammuthus. They lived from the late Miocene epoch (from around 6.2 million years ago) into the Holocene until about 4,000 years ago, with mammoth species at various times inhabiting Africa, Europe, Asia, and North America. Mammoths are distinguished from living elephants by their (typically large) spirally twisted tusks and in at least some later species, the development of numerous adaptions to living in cold environments, including a thick layer of fur.

Mammoths and Asian elephants are more closely related to each other than they are to African elephants. The oldest mammoth representative, Mammuthus subplanifrons, appeared around 6 million years ago during the late Miocene in what is now southern and Eastern Africa. Later in the Pliocene, by about three million years ago, mammoths dispersed into Eurasia, eventually covering most of Eurasia before migrating into North America around 1.5–1.3 million years ago, becoming ancestral to the Columbian mammoth (M. columbi). The woolly mammoth (M. primigenius) evolved about 700–400,000 years ago in Siberia, with some surviving on Russia's Wrangel Island in the Arctic Ocean until as recently as 4,000 years ago, still extant during the existence of the earliest civilisations in ancient Egypt and Mesopotamia.

According to The American Heritage Dictionary, the word "mammoth" likely originates from *mān-oŋt, a word in the Mansi languages of western Siberia meaning "earth horn", in reference to mammoth tusks. Mammoths appear in the folkore of the indigenous people of Siberia, who were impressed by the great size of their remains. In the mythology of the Evenk people, mammoths were responsible for the creation of the world, digging up the land from the ocean floor with their tusks. The Selkup believed that mammoths lived underground and guarded the underworld, while the Nenets and the Mansi (the latter of whom, along with the Khanty, conceived mammoths as giant birds) believed that mammoths were responsible for the creation of mountains and lakes, while the Yakuts regarded mammoths as water spirits.

The word mammoth was first used in Europe during the early 17th century, when referring to maimanto tusks discovered in Siberia, as recorded in the 1618 edition of the Dictionariolum Russico-Anglicum. The earliest scientific research paper on mammoths was by Vasily Tatishchev in 1725. John Bell, who was on the Ob River in 1722, said that mammoth tusks were well known in the area. They were called "mammon's horn" and were often found in washed-out river banks. Bell bought one and presented it to Hans Sloan who pronounced it an elephant's tooth.

In the American colonies around 1725, enslaved Africans digging in the vicinity of the Stono River in South Carolina unearthed molar teeth recognised in modern times to belong to Columbian mammoths, with the remains subsequently examined by the British naturalist Mark Catesby, who visited the site, and later published an account of his visit in 1843. While the slave owners were puzzled by the objects and suggested that they originated from the great flood described in the Bible, Catesby noted that the slaves unanimously agreed that the objects were the teeth of elephants similar to those from their African homeland, to which Catesby concurred, marking the first technical identification of any fossil animal in North America.

In 1796, French biologist Georges Cuvier was the first to identify woolly mammoth remains not as modern elephants transported to the Arctic, but as an entirely new species. He argued this species had gone extinct and no longer existed, a concept that was not widely accepted at the time. Following Cuvier's identification, German naturalist Johann Friedrich Blumenbach gave the woolly mammoth its scientific name, Elephas primigenius, in 1799, placing it in the Elephas, the genus which today contains the Asian elephant (Elephas maximus). Originally the African elephants, as well as the American mastodon (described in 1792) were also placed in Elephas. Cuvier coined the synonym Elephas mammonteus for the woolly mammoth a few months later, but E. primigenius became the widely used name for the species, including by Cuvier. The genus name Mammuthus was coined by British anatomist Joshua Brookes in 1828, as part of a survey of his museum collection.

Thomas Jefferson, who famously had a keen interest in paleontology, is partially responsible for transforming the word mammoth from a noun describing the prehistoric elephant to an adjective describing anything of surprisingly large size. The first recorded use of the word as an adjective was in a description of a large wheel of cheese (the "Cheshire Mammoth Cheese") given to Jefferson in 1802.

The earliest known proboscideans, the clade that contains the elephants, arose about 55 million years ago on the landmass of Afro-Arabia. The closest relatives of the Proboscidea are the sirenians and the hyraxes. The family Elephantidae arose by million years ago in Africa, and includes the living elephants and the mammoths. Among many now extinct clades, the mastodon is only a distant relative of the mammoths, and part of the separate Mammutidae family, which diverged 25 million years before the mammoths evolved.

Following the publication of the woolly mammoths mitochondrial genome sequence in 1997, it has since become widely accepted that mammoths and Asian elephants share a closer relationship to each other than either do to African elephants.

The following cladogram shows the placement of the genus Mammuthus among other proboscideans, based on hyoid characteristics and genetics:

It is possible to reconstruct the evolutionary history of the genus through morphological studies. Mammoth species can be identified from the number of enamel ridges/lamellae on their molars; the primitive species had few ridges, and the amount increased gradually as new species evolved and replaced the former ones. At the same time, the crowns of the teeth became longer, and the skulls became higher from top to bottom and shorter from the back to the front over time to accommodate this.

The earliest mammoths, assigned to the species Mammuthus subplanifrons, are known from southern and eastern Africa, with the earliest records dating to the Late Miocene, around 6.2–5.3 million years ago. By the Late Pliocene, mammoths had become confined to the northern portions of the African continent with remains from this time assigned to Mammuthus africanavus. During the Late Pliocene, by 3.2 million years ago, mammoths dispersed into Eurasia via the Sinai Peninsula. The earliest mammoths in Eurasia are assigned to the species Mammuthus rumanus. The youngest remains of mammoths in Africa are from Aïn Boucherit, Algeria dating to the Early Pleistocene, around 2.3–2 million years ago (with a possible later record from Aïn Hanech, Algeria, dating to 1.95–1.78 million years ago).

Mammuthus rumanus is thought to be the ancestor of Mammuthus meridionalis, which first appeared at the beginning of the Pleistocene, around 2.6 million years ago. Mammuthus meridionalis subsequently gave rise to Mammuthus trogontherii (the steppe mammoth) in Eastern Asia around 1.7 million years ago. Around 1.5–1.3 million years ago, M. trogontherii crossed the Bering Land Bridge into North America, becoming ancestral to Mammuthus columbi (the Columbian mammoth). At the end of the Early Pleistocene Mammuthus trogontherii migrated into Europe, replacing M. meridionalis around 1–0.8 million years ago. Mammuthus primigenius (the woolly mammoth) had evolved from M. trogontherii in Siberia by around 600,000–500,000 years ago, replacing M. trogontherii in Europe by around 200,000 years ago, and migrated into North America during the Late Pleistocene.

A number of dwarf mammoth species, with small body sizes, evolved on islands as a result of insular dwarfism. These include Mammuthus lamarmorai on Sardinia (late Middle-Late Pleistocene), Mammuthus exilis on the Channel Islands of California (Late Pleistocene), and Mammuthus creticus on Crete (Early Pleistocene).

Like living elephants, mammoths typically had large body sizes. The largest known species like Mammuthus meridionalis and Mammuthus trogontherii (the steppe mammoth) were considerably larger than modern elephants, with mature adult males having an average height of approximately 3.8–4.2 m (12.5–13.8 ft) at the shoulder and weights of 9.6–12.7 tonnes (21,000–28,000 lb), while exceptionally large males may have reached 4.5 m (14.8 ft) at the shoulder and 14.3 tonnes (31,526.1 lb) in weight. However, woolly mammoths were considerably smaller, only about as large as modern African bush elephants with males around 2.80–3.15 m (9 ft 2.2 in – 10 ft 4.0 in) high at the shoulder, and 4.5–6 tonnes (9,900–13,200 lb) in weight on average, with the largest recorded individuals being around 3.5 m (11.5 ft) tall and 8.2 tonnes (18,077.9 lb) in weight. The insular dwarf mammoth species were considerably smaller, with the smallest species M. creticus estimated to have a shoulder height of only around 1 metre (3.3 ft) and a weight of about 180 kilograms (400 lb), making it one of the smallest elephantids known.

The number of lamellae (ridge-like structures) on the molars, particularly on the third molars, substantially increased over the course of mammoth evolution. The earliest Eurasian species M. rumanus have around 8-10 lamellae on the third molars, while Late Pleistocene woolly mammoths have 20-28 lamellae on the third molars. These changes also corresponded with reduced enamel thickness and increasing tooth height (hypsodonty). These changes are thought to be adaptations to increasing abrasion resulting from the shift in the diet of mammoths from a browsing based diet in M. rumanus, towards a grazing diet in later species.