In many countries, a mayor is the highest-ranking official in a municipal government such as that of a city or a town. Worldwide, there is a wide variance in local laws and customs regarding the powers and responsibilities of a mayor as well as the means by which a mayor is elected or otherwise mandated. Depending on the system chosen, a mayor may be the chief executive officer of the municipal government, may simply chair a multi-member governing body with little or no independent power, or may play a solely ceremonial role. A mayor's duties and responsibilities may be to appoint and oversee municipal managers and employees, provide basic governmental services to constituents, and execute the laws and ordinances passed by a municipal governing body (or mandated by a state, territorial or national governing body). Options for selection of a mayor include direct election by the public, or selection by an elected governing council or board.

The term mayor shares a linguistic origin with the military rank of major, both ultimately derived from French majeur, which in turn derives from Latin maior, the comparative form of the adjective magnus.

In modern England and Wales, the position of mayor descends from the feudal lord's bailiff or reeve (see borough). The chief magistrate of London bore the title of portreeve for considerably more than a century after the Norman Conquest. This official was elected by popular choice, a privilege secured from King John. By the beginning of the 12th century, the title of portreeve gave way to that of mayor as the designation of the chief officer of London, followed around 1190 by that of Winchester. Other boroughs adopted the title later.^{[citation needed]}

In the 19th century, the Municipal Corporations Act 1882, Section 15, regulated the election of mayors. The mayor was to be a fit person elected annually on 9 November by the council of the borough from among the aldermen or councilors or persons qualified to be such. His term of office was one year, but he was eligible for re-election. He might appoint a deputy to act during illness or absence, and such deputy must be either an alderman or councilor. A mayor who was absent from the borough for more than two months became disqualified and had to vacate his office. A mayor was ex officio a justice of the peace for the borough during his year of office and the following year. He received such remuneration as the council thought reasonable. These provisions have now been repealed.

In medieval Wales, the Laws of Hywel Dda codified the mayor (Latin: maior; Welsh: maer) as a position at the royal courts charged with administering the serfs of the king's lands. To maintain its dependence on and loyalty to the Crown, the position was forbidden to the leaders of the clan groups. A separate mayor, known as the "cow dung mayor" (maer biswail), was charged with overseeing the royal cattle. There were similar offices at the Scottish and Irish courts.^{[citation needed]}

The office of mayor in most modern English and Welsh boroughs and towns did not in the 20th century entail any important administrative duties and was generally regarded as an honor conferred for local distinction, long service on the council, or for past services. The mayor was expected to devote much of their time to civic, ceremonial, and representational functions, and to preside over meetings for the advancement of the public welfare. The administrative duties of mayors were to act as returning officer at parliamentary elections and to chair the meetings of the council.

The mayor of a town council is officially known as "town mayor" (although in popular parlance, the word town is often dropped). The person is known as "mayor" regardless of gender; the wife of a male mayor is traditionally known as the "mayoress". Since the 1974 local government reforms, mayors are also appointed to English local government districts which have borough status. This results in mayors of districts which include towns which also have separate mayors. In districts which do not have borough status, the role of civic leader is taken by the chairman of the council, who undertakes exactly the same functions as a mayor of a district with borough status.

Many major cities in the United Kingdom and throughout the Commonwealth have a lord mayor, a special recognition bestowed by the sovereign. Although the position is often ceremonial, with executive responsibilities under the control of an elected mayor, lord mayors in several Commonwealth cities including Belfast, Cardiff, Brisbane, and Dublin hold both ceremonial and executive duties. Some lord mayors, including the Lord Mayor of Sydney (Australia), are elected.