Mount Gambier is the second most populated city in South Australia, with an estimated urban population of 33,233 as of 2021. The city is located on the slopes of Mount Gambier, a volcano in the south east of the state, about 450 kilometres (280 mi) south-east of the capital Adelaide and just 17 kilometres (11 mi) from the Victorian border. The traditional owners of the area are the Bungandidj (or Boandik) people. Mount Gambier is the most important settlement in the Limestone Coast region and the seat of government for both the City of Mount Gambier and the District Council of Grant.

In the 2021 Census the population of the Mount Gambier census area was 25,591 people, making it the largest urban area in the state outside Adelaide. Approximately 52% of the population were female, 82.8% were Australian born, over 91.5% of residents were Australian citizens and 2.8% were indigenous.

Mount Gambier sits on a number of highways which connect the city to other major towns in the region, as well as to Adelaide and Melbourne.