Thomas Mark Barrett is an American diplomat and politician who has served as the United States ambassador to Luxembourg since 2022. He previously served as the 44th mayor of Milwaukee, Wisconsin from 2004 until 2021.

Milwaukee is the most populous city in the U.S. state of Wisconsin and the county seat of Milwaukee County. With a population of 577,222 at the 2020 census, Milwaukee is the 31st-most populous city in the United States, and the fifth-most populous city in the Midwest. It is the central city of the Milwaukee metropolitan area, the 40th-most populous metro area in the U.S. Milwaukee is categorized as a "Gamma minus" city by the Globalization and World Cities Research Network, with a regional GDP of over $102 billion in 2020.