Sumter County was established by the Florida Legislature on January 8, 1853. It was named for Revolutionary War hero Gen. Thomas Sumter (1736-1832), a native of South Carolina who was prominent in the southern campaign of the Revolutionary War. Many South Carolinians were early settlers in this area. Sumter county was once the southern portion of Marion County and, for a time, contained the land that is now western lake County. The area had been settled for several decades by the time the Legislature chartered it as the states 29th county.
After the state Legislature took a portion of Sumter and Orange counties to form Lake County an election in 1881 established Sumterville as the new county seat.
The County courthouse in Sumterville was destroyed by fire in 1909. The loss of the courthouse along with nearly two decades of county records set off a round of political infighting that eventually led to a 1912 county-wide vote to establish a new County seat. Votes were cast between the towns of Wildwood and Bushnell. By a margin of only nine, Bushnell was selected the new County Seat for Sumter County - Bushnell 657, Wildwood 648 votes.
Much of what is now eastern Sumter County was part of the original Seminole Indian reservation established under the Treaty of Moultrie in 1824. As a result, the area played an extremely important role in the Second Seminole War.
The Second Seminole War, in fact, began when two companies of US Army infantry under the command of Maj. Francis Dade were attacked by a party of Seminoles on Dec. 28, 1835. The US troops were en route from Ft. Brooke (near Tampa) to Ft. King (today's Ocala) when they were ambushed near present-day Bushnell. Only two of the 108 Army troops escaped the battle, although one of them later died of his wounds.
Dade Battlefield Historic State Park now marks the site of this historic battle. There is a museum located at the park, and monuments and plaques mark the course of the battle. Each year on the battles anniversary a reenactment of the battle attracts both local spectators and history enthusiasts.
In 1860, the county's first census showed a population of 1,429. Early inhabitants were farmers and citrus growers. In the Secession Convention of 1861, Sumter County Representative David G. Leigh voted to leave the union.
By 1886 there were over 100 orange growers in the county. The freeze of 1894-95 practically destroyed the citrus industry. Many of the farmers converted to cattle ranching, and as a result the county's population nearly doubled within ten years. The cattle industry became the most important industry rivaled only by the vegetable industry.
One of the earliest towns established in Sumter County was Adamsville. Official county business was conducted in Leesburg (then a part of the county), but Adamsville played an important role in the areas commerce because of its proximity to the railroad, stage lines and the telegraph road through the area.