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Round Rock and Williamson County have been the site of human habitation since at least 9,200 BC. The area's earliest known inhabitants lived during the late Pleistocene (Ice Age), and are linked to the Clovis culture around 9,200 BC based on evidence found at the much-studied "Gault Site", midway between Georgetown and Fort Hood. One of the most important discoveries in recent times is the ancient skeletal remains dubbed "the Leanderthal Lady" because of its age and proximity to Leander, Texas. The site is 4 miles (6 km) west of Round Rock and was discovered by accident by Texas Department of Transportation workers while drilling core samples for a new highway. The site has been studied for many years and samples carbon date to this particular Pleistocene period around 10,500 years ago.

Prehistoric and Archaic period "open occupation" campsites are also found throughout the county along streams and other water sources, including Brushy Creek in Round Rock and the San Gabriel River in Georgetown, 10 mi (16 km) north. These archeology dig sites show a much greater volume United States evidence of Archaic period inhabitants based on relics and flint tools recovered from burned rock middens. The earliest known "historical" Native American occupants, the Tonkawa, were a flint-working, hunting people who followed the buffalo on foot and periodically set fire to the prairie to aid them in their hunts.

During the 18th century, the Tonkawa made the transition to a horse culture and used firearms to a limited extent. Apparently, small numbers of Kiowa, Yojuane, Tawakoni, and Mayeye Native-Americans lived in the county at the time of the earliest Anglo settlements. After they were crowded out by white settlement, the Comanches raided settlements in the county until the 1860s. In the late 19th century, Native Americans were being pushed out of Central Texas.

As the area developed into a rural Anglo community, some of the modern paved roads followed the original Native-American pathways. One famous immigration route passed through Round Rock and is called the "Double File Trail" because the path was wide enough for two horsemen to ride side-by-side. It is part of a longer trail from North Texas that crossed the San Gabriel River in Georgetown, Brushy Creek in Round Rock, and the Colorado River in Austin. An elementary school in the Round Rock school district is named for the trail, Double File Trail Elementary School.

In 1851, a small community was formed on the banks of Brushy Creek, near a large round and anvil-shaped rock in the middle of the creek. This round rock marked a convenient low-water crossing for wagons, horses, and cattle. The first postmaster called the community "Brushy", and the creek was called "Brushy Creek", but in 1854, at the suggestion of the postmaster, the small settlement was renamed Round Rock in honor of this now famous rock. After the Civil War, Jesse Chisholm began moving cattle from South Texas through Round Rock on the way to Abilene, Kansas. The route he established, which crossed Brushy Creek at the round rock, became known as the Chisholm Trail. Most of the old buildings, including the old Saint Charles Hotel, have been preserved. This historic area is now called "Old Town".

Downtown Round Rock was the site of a historic gunfight and subsequent capture (and death) of the 19th-century American train robber Sam Bass, by the Texas Ranger Division on July 19, 1878. The Rangers followed Bass and his gang after they robbed the Fort Worth-to-Cleburne train. Bass was tracked to Round Rock, and as he attempted to flee, Bass was shot and killed in a gun battle by Ranger George Herold and Ranger Sergeant Richard Ware. Sheriff's Deputy A.W. Grimes was killed in the shootout. Near Ware was Soapy Smith, a noted con man, and his cousin Edwin, who witnessed Ware's shot. Soapy exclaimed, "I think you got him." The event is known locally as the "Sam Bass Shootout". This shootout is recreated each year at the July 4 Frontier Days Celebration in Old Settlers Park. Bass is buried in Round Rock Cemetery, northwest of "Old Town" on Sam Bass Road. His original headstone can be found on display at the Round Rock Public Library.

In the first half of the 20th century, the county's wealth came from the cotton fields.Cotton, row crops, grapes, and truck farming were the predominant subsistence east of Interstate 35. West of the Balcones divide, ranchers raised cattle, sheep, and to a lesser extent, goats. Due to Round Rock's favorable geographic location over the rich, fertile "blackland prairie" soils also known locally as the "black waxy" (due to the soil's high clay content), cotton was the largest economic driver at that time. Because of the soil and climate, this ecoregion is ideally suited to crop agriculture. Nearby Taylor, Texas, east of Round Rock, was the primary cotton center where the crop was hauled for ginning (its seeds mechanically removed) at the cotton gin, compressed into bales, and shipped by train. Austin was also a cotton center for a time once the railroad arrived there in the 1870s. Cotton production and cattle raising, on a much smaller scale, continues today, although primarily east of Round Rock.

To preserve the heritage of the famous crossing, a Chisholm Trail Crossing Park was developed to provide visitors with a simulated scene of Round Rock's historical role in the Chisholm cattle drive. Commemorative plaques in the park tell of the history of Round Rock. The bronze sculptures of four steers and pioneer woman Hattie Cluck and her son, Emmitt, were commissioned by the city through donations from Round Rock residents. The sculptures depict Round Rock's history as a crossing location along the Chisholm Trail. The project plans include 18 to 20 additional bronze statues over time.

Following the end of the American Civil War, a group of Confederate veterans held a reunion in Georgetown on August 27, 1904, for the old settlers of Williamson County and their descendants. The invitation promised "good music, plenty to eat, and above all a warm welcome." The event was well-attended, and reunions — now called Old Settlers Association (OSA) reunions — have been held annually ever since. After the initial one, the event was moved to Round Rock and eventually a structure was built (along with three restored log cabins) in the Palm Valley area of Round Rock, in front of Old Settlers Park, just off Highway 79 in east Round Rock. All members of the organization are descendants of Williamson County residents prior to 1904. OSA has about 50 active members and 300 members in all. The Old Settlers Association today is a social and educational group, with the purpose of facilitating social activities, as well as collecting and preserving important historical information and facts. The facilities are rented for meetings, arts and craft and collectable shows, events, parties, weddings and rehearsal dinners.

In the 1950s, planners of the new Interstate Highway System proposed to route Interstate 35 through Taylor, whose population and cotton industry made it the county's economic powerhouse. Highway Commissioner DeWitt Greer called for the "interregional" highway to go through Taylor on its way from Dallas to Austin, but some Taylor leaders and other citizens fought the idea, worried about the possibility of cutting farmers off from all or part of their fields, traffic noise, damage to country life, loss of farmland, and unwanted right-of-way acquisition — it was proposed to be an astounding 300 feet (90 m) wide, unheard of before this time. No one even knew what an "Interregional Highway" would look like, unless they had traveled to Germany to see the Autobahn or the Merritt Parkway in Connecticut. Instead, they wanted improvements to the farm-to-market roads and a straight route to Austin.

Meanwhile, Round Rock leaders sought the highway and its potential economic benefits. Mayor Louis Henna lobbied the Highway Commission. In June 1956, the 15-year debate over the form, funding, and route of the Interstate was resolved. Due to the heavy lobbying, and not wanting to antagonize Taylor, the highway was built along the edge of the Balcones Fault line, running through Round Rock. The precise route was not without opposition, however, as the new road cut off "Old Town" to the west from what had become the more recent "downtown" area east of Interstate 35. The Interstate eventually made Round Rock into a viable and vibrant commercial center, while Taylor withered with the decline of the cotton industry. Today, it is a minor, modest town with a smaller population, while Round Rock has thrived and rapidly grown into the largest city in the county, attracting Dell Computer and major retail centers. The transformation of Round Rock is detailed in a book by Linda Scarborough (publisher of the Williamson County Sun newspaper) titled Road, River and Ol' Boy Politics: A Texas County's Path from Farm to Supersuburb published by Texas State Historical Press.

By the 1990s, Round Rock was primarily a bedroom community with the majority of its employed residents working in Austin and then returning home after work to places such as Round Rock and Georgetown, where housing and land were less expensive. In the 1990s, Round Rock had few major employers and jobs other than local retail and other services, or ranching and farming. In the late 1990s, though, that began to change as economic development became a major focus of the city and the Chamber of Commerce. Dell Corporation moved its headquarters to Round Rock, which has provided a significant number of jobs with 16,000 employees at its Round Rock headquarters.(See also the Business and economic development section in this article.)

As of the 2010 census, 99,887 people and 37,223 households were residing in the city. The 37,223 housing units had 20,364 owner-occupied homes with a median value of $163,400. The racial makeup of the city was 76.4% White, 9.4% African American, 0.4% Native American, 4.1% Asian, 5.7% from other races, and 3.0% from two or more races. Hispanics or Latinos of any race made up 25% of its population. According to a 2009 estimate by the U.S Census Bureau, the median income for a household was $69,892, and for a family was $79,417.

Of the 21,076 households, 47.5% had children under the age of 18 living with them, 60.5% were married couples living together, 11.0% had a female householder with no husband present, and 24.4% were not families. About 18.1% of all households were made up of individuals, and 3.0% had someone living alone who was 65 years of age or older. The average household size was 2.87, and the average family size was 3.29.

In the city, the age distribution was 31.9% under 18, 8.5% from 18 to 24, 38.8% from 25 to 44, 16.3% from 45 to 64, and 4.5% who were 65 or older. The median age was 30 years. For every 100 females, there were 99.1 males. For every 100 women age 18 and over, there were 96.3 men.

The per capita income for the city was $24,911.