[3] The county is named for Neil McLennan,[4] an early Scottish settler who worked to evict the Native Americans in frontier Texas.

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.

In the aftermath of World War I, when social tensions were high as veterans returned, white racial violence broke out against blacks.

County resident Doris Miller was awarded the Navy Cross for his heroism at Pearl Harbor; he was the first African American to earn such distinction.

Completed in 1902 in the Beaux-Arts style, it is the next-to-last example of architect James Riely Gordon's Texas courthouses.

Crush, Texas, was a temporary "city" in McLennan County, about 15 miles (24 km) north of Waco.

After strong promotion, on September 15, 1896, the event was delayed by an hour as the police maneuvered the crowd of more than 40,000 back to what was thought to be a safe distance.

The two engines, pulling wagons filled with railroad ties, traveled a 4-mile (6.4 km) track and thunderously crashed into each other at a combined speed up to 120 mph (190 km/h).

Three people were killed and about six were injured, including event photographer Jarvis "Joe" Deane, who lost an eye because of a flying bolt.

[7] Texas composer and singer Brian Burns wrote and recorded a song about the collision, "The Crash at Crush" (2001).

The 2020 census showed on a heat map that McLennan County displayed an estimated 1.3% of partnered households are in same-sex relationships, though may be undercounted.

Several governors of Texas hailed from McLennan County: Ann Richards (1991–1995), the state's second female governor; Pat Morris Neff (1921–1925), who also served as president of Baylor University; Lawrence Sullivan Ross (1887–1891), whose family helped found Waco and who also served as the fourth president of Texas A&M University; and Richard Coke (1874–1876).