Keller was born in Dallas on August 1, 1953. Her father, Jack Keller, started Keller's Drive-In in 1950; there were eventually three restaurants, though one has since closed. Sharon had two brothers and an older sister and went to the exclusive private Greenhill School, graduating in 1970. She spent a year at the conservative Catholic college the University of Dallas, then, at her father's suggestion, transferred to Rice University, where her sister was attending school. Sharon was reserved and withdrawn, though according to several friends of hers from the time, she was no straight arrow and availed herself of the freedoms of the counterculture.

In a 1998 interview, Keller said, "I don't think people who knew me before I took the bench would perceive me as a leader-type person." Not only that, friends don't remember her even talking about big issues of the day, such as the death penalty, which was about to be outlawed, or the Vietnam War. "Most of our discussions were about where to go to have fun," remembers one friend. "It would be a shock to most people that she would go to law school, much less become the most conservative Court of Criminal Appeals judge in the state's history."

Like many young people, Keller didn't know what to do with her life. At first, she planned to study physics but later changed her major to philosophy. Her father suggested law, and after graduating from Rice, she enrolled at Southern Methodist University, earning her degree in 1978. She worked at a small firm in private practice from 1978-1981 and married Hunt Batjer, a neurosurgeon. After their son, Temple, was born, she worked from 1981-1987 for her father's company as a senior executive. She and Batjer divorced soon afterward. In 1987 a parent of
one of Temple's Greenhill schoolmates told Keller about an opening at the Dallas County DA's office; she applied and got it. As an assistant DA, she worked in the appellate division, and one of her jobs was arguing before the CCA, In 1993 Keller's boss, DA John Vance, suggested that she run for a CCA seat that had come open at the last minute in the upcoming election.

Keller campaigned as a tough-on-crime district attorney who would bring "the perspective of a prosecutor" to the court and not let criminals off on minor procedural violations. She outspent her general election foe by a three-to-one margin--Keller borrowed $213,000 from her family and raised another $25,000--and won easily, carried onto the court by the wave of anti-Clinton, pro-Bush sentiment, becoming the first woman elected to the court.

In 2000 and 2006, she was challenged in the Republican primary for the position of presiding judge by another member of the CCA, Judge Tom Price, who said she had made the CCA a "national laughingstock". She narrowly beat Price both times.