The county was originally to have been named San Gabriel County, but was instead named after Robert Mc Alpin Williamson (Three-Legged Willie), a Texas statesman and judge at the time.
Georgetown was an agrarian community for most of the 19th and early 20th centuries.
Population growth and industrial expansion continued modestly in the 20th century until about 1960, when residential, commercial, and industrial development, due to major growth and urban expansion of nearby Austin, greatly accelerated.
One of the most important discoveries in recent times is that of the ancient skeletal remains dubbed "The Leanderthal Lady" because of its age and proximity to nearby community Leander, Texas.
The site is immediately southwest of Georgetown and was discovered by accident by Texas Department of Transportation workers while core samples for a new highway were being drilled.
Georgetown was named for George Washington Glasscock who donated the land for the new town.
Early American and Swedish pioneers were attracted to the area's abundance of timber and good, clear water. Georgetown is the county seat of Williamson County, which was formed on March 13, 1848, after the early settlers petitioned the state legislature to create it out of Milam County.