The Romans, who invaded Britain in A. D. 43, first founded London (which they called Londinium) at the site of the present-day City of London (the oldest, walled part or the "square mile") on the northern bank of the Thames River. Although burned down in a rebellion a scant 17 years later, the city was soon rebuilt and had become a flourishing trading center by A. D. 100.

Hyde Park is surrounded by London's fashionable, stylish, and aristocratic neighborhoods. ()
By the middle of the third century, Londinium was the largest city in Britain, with a population of as many as 50,000 inhabitants, and its boundaries corresponded to those of today's historic central core. In the fifth century, the Romans, under siege by Germanic invaders, vacated Londinium, and the city entered a long period of decay and neglect.

Following a Danish invasion in 878, King Alfred of Wessex (849–899) retook and began rebuilding the city, which expanded northward and became known as Londontown. South-wark, on the south bank of the Thames, also grew and prospered, and Westminster, upstream from London and at that time an island surrounded by marsh-land, underwent development when King Edward the Confessor (c. 1003–1066) built a royal palace there following his accession in 1042. This was the beginning of Westminster's history as home to royalty and center of government. It was in Westminster Abbey that William, Duke of Normandy (1027–1087), was crowned king of England following the Norman Conquest in 1066. To win the cooperation of London's political leaders and wealthy merchants, he granted the city special powers through a charter.

By the end of the twelfth century, London had a population of around 40,000 and had elected its first mayor. In the fourteenth century, disaster struck, in the form of the Black Death, which spread to London via ships traveling from Europe and ultimately killed about one-third of England's population. Over the next three centuries, London was to undergo several recurrences of the epidemic.

The inception of the Tudor dynasty in 1485 brought the city further growth and prosperity, peaking with the reign of Elizabeth I in the sixteenth century, by which time London was the center of a global empire and one of the foremost cities of Europe. In the meantime, the population outside the city walls had grown dramatically, reaching 200,000 by the year 1600. Decrees were issued to slow further growth, limiting London to a "Green Belt" surrounding the outer city. The restrictions caused overcrowding in the central city, contributing to a new outbreak of plague in 1665. The following year, roughly two-thirds of the city (by now the world's largest) was burned down in the Great Fire.

A massive rebuilding effort restored the city, with brick and stone replacing its original wooden buildings. A new grid-based plan for the city by architect Sir Christopher Wren was not adopted, however, and London's layout essentially retained its original patterns. London grew rapidly in the eighteenth century, with a steadily expanding population and new streets and neighborhoods. Its first squares, including Covent Garden and Leicester Square, added new elegance to the fashionable parts of town. However, many in the oldest districts lived in dire poverty, and crime and rioting were commonplace.

In the nineteenth century, the Industrial Revolution caused further deterioration in living conditions for many Londoners, polluting their air and worsening the already existing pollution of the Thames. Yet, London remained the largest and wealthiest city in the world and the center of a vast empire, and the Great Exhibition of 1851, held in the Crystal Palace constructed in Hyde Park, celebrated the city's achievements.

The two world wars of the twentieth century brought air raids to London; those of World War II left the city decimated and necessitated large-scale rebuilding. A major postwar development was the exodus of many Londoners to the suburbs. The drive to attract immigrant labor from Britain's former Third World colonies turned London into a multi-ethnic city but also led to racial tensions.

Recent decades brought the "Swinging 60s," when London became the world's capital of popular culture; the economic crises of the 1970s; and the Thatcher Era of the 1980s, when the Greater London Council (GLC) was abolished, leaving the city with no metropolitan government. As the twentieth century drew to a close, London was on the eve of a new era in local government as its citizens prepared to elect a mayor and council.