Zellweger was born on April 25, 1969, in Katy, Texas. Her father, Emil Erich Zellweger, is from the Swiss town of Au, St. Gallen and descends from an Appenzell family, and was a mechanical and electrical engineer who worked in the oil refining business. Her mother, Kjellfrid Irene (née Andreassen), is Norwegian. Kjellfrid grew up in Ekkerøy near Vadsø, as well as Kirkenes, and was a nurse and midwife who moved to the United States to work as a governess for a Norwegian family in Texas. Zellweger has described herself as being raised in a family of "lazy Catholics and Episcopalians".
Zellweger attended Katy High School, where she was a cheerleader, gymnast, speech team member, and drama club member. She also participated in soccer, basketball, baseball, and football. In 1986, her academic paper, "The Karankawas and Their Roots", won third place in the first ever Houston Post High School Natural Science Essay Contest. After high school, she enrolled at the University of Texas at Austin, where she graduated with a Bachelor of Arts degree in English in 1991. While at university, Zellweger took a drama course as an elective, which initially sparked her interest in acting.
In Zellweger's junior year, her father lost his job and was unable to support her at college, so she took a job as a cocktail waitress in Austin, Texas. Zellweger said of the job, "I learned a lot. As much as I did in my classes that that club paid for... I learned not to judge people, [and] that things are not black and white." Zellweger began getting small parts acting, and earned her Screen Actors Guild card for doing a Coors Light commercial. Also while in college, she did "a bit part ... as a local hire" in the Austin-filmed horror-comedy film My Boyfriend's Back, playing "the girl in the beauty shop, maybe two lines. But the beauty shop [scene] got cut." Her first job after graduation was working in a beef commercial, while simultaneously auditioning for roles around Houston, Texas.