The Mayor of the City of Saint John is the head of the elected municipal council of Saint John, New Brunswick, Canada. Saint John Common Council consists of a mayor (chair) and 10 councillors. The city has a ward system with four of roughly equal population. This municipal arrangement was first adopted in the 2008 municipal election after a plebiscite held October 9, 2007. The mayor is directly elected along with two councillors that run at large. Since 2021, the deputy mayor has been chosen by Common Council after each election, having previously been the councillor at large who gained a plurality of votes. The current deputy mayor is John MacKenzie. Each ward elects two councillor.

Since the incorporation of Saint John in 1785, the mayor has served as the chief representative of the city. Officeholders were first appointed by the Governor of New Brunswick, under an order in council. By the end of the end of the nineteenth century, the mayor was elected by the alderman of Common Council. In 1854, the office was directly elected by Saint John residents. As head of the largest city in New Brunswick, many mayors have served as elected officials at the federal and provincial level both before and after their tenure.

Since 1785, 67 individuals have held the office of mayor. Notably, Samuel Davis, 1977-1980, served as the first elected Jewish Mayor of Saint John. In 1983 Elsie Wayne was elected the first woman mayor. She served until 1993. While serving only two years, Thomas J. Higgins was the first Catholic Mayor of Saint John. The former, Don Darling, was elected May 9, 2016. Donna Reardon, incumbent, was elected May 25, 2021.