Zellner started her law firm in January 1991. The firm represents both civil and criminal clients. She has obtained the exoneration of 17 wrongfully convicted men, handling many of these cases pro bono.[4] She obtained the release of death row inmate Joseph Burrows by persuading the real killer, Gayle Potter, to confess to the murder. The Illinois Supreme Court affirmed the trial court decision releasing Burrows from death row.[5]
In November 2009, Zellner took on client Ryan Ferguson, who had served seven years in prison following a conviction for murder.[6][7] She gained an overturn of his conviction by the Missouri Court of Appeals Western District.[8] On November 12, 2013, Ferguson was released from prison after serving 10 years.
In 1994, Zellner persuaded Larry Eyler to confess to 21 murders prior to his death. All 21 murder cases were closed.
She has won millions of dollars in civil jury trials and settlements.[9] She also won $15.5 million for the false arrest and malicious prosecution of Kevin Fox, who was incarcerated for 8 ½ months for the murder of his 3-year-old daughter.[10] His wife, Melissa, recovered for her loss of consortium and intentional infliction of emotional distress. On appeal, the appellate court reduced the verdict to $8.1 million.[11]