Lamar is a city in and the county seat of Barton County, Missouri, United States. As of the 2020 census, the city population was 4,266. It is kn…Lamar is a city in and the county seat of Barton County, Missouri, United States. As of the 2020 census, the city population was 4,266. It is known as the birthplace of Harry S. Truman, the 33rd president of the United States.