McLennan County was created by the Texas Legislature in 1850 out of Milam County, and is named for Neil McLennan, an early settler. The county seat, Waco, had been founded originally as an outpost of the Texas Rangers, laid out by George Erath, and was known by 1850 as "Waco Village."
According to local lore, the first sustained flight did not occur in Kitty Hawk, North Carolina, but just outside Tokio, a small community in McLennan County, by a man flying a gyrocopter.
During World War I, McLennan County was home to at least 1 military airfield, Rich Field. McLennan County's contributions to World War II include the reopening of Rich Field, Doris Miller (awarded the Navy Cross for his heroism at Pearl Harbor, also the first African American to earn such distinction), and James Connally (a locally famous World War II fighter pilot).
Currently, McLennan County includes 22 incorporated cities and 2 unincorporated over a span of 1060 square miles with a population of 260,579 according to the 2020 census.