The island was first inhabited by people who crossed over the land bridge from the European mainland. Traces of early humans have been found (at Boxgrove Quarry, Sussex) from some 500,000 years ago and modern humans from about 30,000 years ago. Until about 10,000 years ago, Great Britain was joined to Ireland, and as recently as 8,000 years ago it was joined to the continent by a strip of low marsh to what is now Denmark and the Netherlands. In Cheddar Gorge near Bristol, the remains of animal species native to mainland Europe such as antelopes, brown bears, and wild horses have been found alongside a human skeleton, 'Cheddar Man', dated to about 7150 BC. Thus, animals and humans must have moved between mainland Europe and Great Britain via a crossing. Great Britain became an island at the end of the Pleistocene ice age when sea levels rose due to isostatic depression of the crust and the melting of glaciers.

According to John T. Koch and others, Britain in the Late Bronze Age was part of a maritime trading-networked culture called the Atlantic Bronze Age that also included Ireland, France, Spain and Portugal where Celtic languages developed.

Its Iron Age inhabitants are known as the Britons, a group speaking a Celtic language. The Romans conquered most of the island (up to Hadrian's Wall, in northern England) and this became the Ancient Roman province of Britannia. For 500 years after the Roman Empire fell, the Britons of the south and east of the island were assimilated or displaced by invading Germanic tribes (Angles, Saxons, and Jutes, often referred to collectively as Anglo-Saxons). At about the same time Gaelic tribes from Ireland invaded the north-west, absorbing both the Picts and Britons of northern Britain, eventually forming the Kingdom of Scotland in the 9th century. The south-east of Scotland was colonized by the Angles and formed, until 1018, a part of the Kingdom of Northumbria. Ultimately, the population of south-east Britain came to be referred to, after the Angles, as the English people.

Germanic speakers referred to Britons as Welsh. This term eventually came to be applied exclusively to the inhabitants of what is now Wales, but it also survives in names such as Wallace, and in the second syllable of Cornwall. Cymry, a name the Britons used to describe themselves, is similarly restricted in modern Welsh to people from Wales, but also survives in English in the place name of Cambria. The Britons living in the areas now known as Wales and Cornwall were not assimilated by the Germanic tribes, a fact reflected in the survival of Celtic languages in these areas into modern times. At the time of the Germanic invasion of Southern Britain, many Britons emigrated to the area now known as Brittany, where Breton, a Celtic language closely related to Welsh and Cornish and descended from the language of the emigrants, is still spoken. In the 9th century, a series of Danish assaults on northern English kingdoms led to them coming under Danish control. In the 10th century, however, all the English kingdoms were unified under one ruler as the kingdom of England. In 1066, England was conquered by the Normans, who introduced a French ruling élite that was eventually assimilated. Wales came under Anglo-Norman control in 1282, and was officially annexed to England in the 16th century.

On 20 October 1604 King James (who had succeeded separately to the two thrones of England and Scotland) proclaimed himself as "King of Great Brittanie, France and Ireland", a title that continued to be used by many of his successors. However, England and Scotland each remained legally in existence as separate countries with their own parliaments until 1707, when each parliament passed an Act of Union to ratify the Treaty of Union that had been agreed the previous year. This had the effect of creating a united kingdom, with a single, united parliament, from 1 May 1707. Though the Treaty of Union referred to the new all-island state, as the "United Kingdom of Great Britain", many regard the term 'United Kingdom' as being descriptive of the union rather than part of its formal name (which the Treaty stated was to be 'Great Britain' without further qualification.) Most reference books, therefore, describe the all-island kingdom that existed between 1707 and 1800 as the "Kingdom of Great Britain".