The land containing Hamilton County was brought into the possession of the United States by the
Treaty of St. Mary's in 1818.
William Conner was the first white settler in the county. In the summer of 1822, after realizing there were enough settlers in the area, Conner and other settlers applied to the Indiana Legislature for a charter authorizing them to become a separate and independent county under Indiana law. The application was presented to the 1822–23 session of the
Indiana General Assembly, and the act was passed and approved by the governor on January 8, 1823. The act took effect on the first Monday in April (April 7), 1823. The county commissioners first met on May 5, 1823, at Conner's house; it also served as the county circuit courthouse. The county was named for
Alexander Hamilton, the first secretary of the US treasury. 2,165 soldiers from Hamilton County served in the Civil War. The last veteran residing in the county died in 1942. Most Belgian immigration to Indiana occurred in the northwestern part of the state in
Lake County and
Porter County. However, in the 1890s, a significant number of Belgian immigrants also settled in the Noblesville area of Hamilton County. Similarly, chain migration led to numerous first generation Serbian immigrants who had settled in Indianapolis, and their Indiana-born children, to move to Arcadia, Carmel, Sheridan, Noblesville, and Cicero around the turn of the 20th century. Like most of Indiana, the area was also heavily
German-American, with Germans being the second largest single ethnic group in the county at the time, second only to so-called "native born whites" whose ancestry extended to the original
Thirteen Colonies and who were the largest ethnic group in Hamilton County at the turn of the century. During
World War I, Hamilton County was noted for an extreme rise in anti-German sentiment. The name of Wilhelm Street in Noblesville was changed to Washington Street and German-Americans were subjected to bullying and abuse from their neighbors. German-Americans were one of the largest ethnic groups in the county, but the remaining two-thirds of the county were "whipped into an anti-German frenzy" during which Germanness and Americanness were presented as being antithetical throughout the press. Hamilton County was labeled the "most anti-German county" in Indiana, although such attitudes were common throughout the rest of Indiana as well. During most of 1940 and 1941, Indiana as a whole was very isolationist, with much of the state preferring that the country stay neutral in the European conflict. However, Hamilton County was a "hot-bed of interventionist and pro-British sentiment", which stood out because unlike many other places in Indiana, most people in Hamilton County were in favor of American entry into the war, and were in favor of providing Britain with as much aid as possible as well. ==Climate and weather==