The powerhouse attorney is famous for overturning wrongful convictions, and gained widespread public notoriety for her work representing Steven Avery shown in Netflix's hit docuseries Making a Murderer.
"I have spent the last 30 plus years defending individuals who have been falsely accused of wrongdoing," Zellner explained in a statement to Page Six.
She continued: "I welcome adding Johnny Depp to that list and joining his dream team who share this view and have so effectively been advocating for him."
The Pirates of the Caribbean actor recently lost his libel lawsuit against a writer in London who referred to him as a "wife-beater" in a headline, but he is still suing the Aquaman star in the United States over a published news story in which Heard called herself a victim of domestic abuse.
"I became a public figure representing domestic abuse," she wrote at the time, "and I felt the full force of our culture's wrath for women who speak out."
As OK! previously reported, The Rum Diary actress requested to have the massive $50 million lawsuit dismissed, but her petition was rejected.
"Mr. Depp never abused Ms. Heard," his legal team stated. "Her allegations against him were false when they were made in 2016. They were part of an elaborate hoax to generate positive publicity for Ms. Heard and advance her career."
After a judge rejected her plea to dismiss the case, the Edward Scissorhands star's lawyer Ben Chew said the actor was "most gratified by the Court's decision," in a statement to Deadline.
As the Charlie and the Chocolate Factory lead's career and reputation continue to be destroyed by the statements made by Heard, Depp's former bandmate Bill "Beano" Hanti pins the blame entirely on Heard.
"Ruined his life, it really did," Hanti said in a clip from the Johnny Vs Amber documentary on Discovery+. "It totally f**ed his career over, cost him his friends, part of his family."
"She got inside John's head," he continued. "And made him absolutely crazy to the point where he was drinking and drugging, to the point of insanity."