She was admitted to the Wisconsin State Bar in 1989. While in law school she was a staff editor of the law review, as well as a recipient of the Dean's Award.
Before serving in the judiciary, Ziegler worked as a Federal Criminal Prosecutor, and an Assistant United States Attorney, in the Eastern District of Wisconsin. She was also a pro bono Special Assistant District Attorney in the Milwaukee County District Attorney’s Office. In private practice, she was a civil private practice attorney for several years at the law firm of O’Neil, Cannon, Hollman & DeJong, SC.
In 1997, Ziegler was appointed by then-Gov. Tommy Thompson to the Washington County Circuit Court. She won a full term to that seat by election in 1998 and was re-elected in 2004. She then chose to run for election to the Supreme Court seat being vacated by retiring Justice Jon P. Wilcox.
The campaign was a contentious one; Clifford contended Ziegler ruled in cases where she was unable to be objective. Ziegler contended Clifford had never been a judge and would be ill-prepared to do so; she also raised concerns about two of Clifford's campaign workers misrepresenting themselves to law enforcement officials. On April 3, 2007, Ziegler defeated Clifford in the election, 58% to Clifford's 42%.