He entered private practice in Seattle, Washington, working from 1962 to 1988 at the law firm Lane Powell Moss & Miller.
[2] In November 1988 Zilly ruled that the US Fish and Wildlife Service had unlawfully failed to list the northern spotted owl under the Endangered Species Act.
[5] In June 2002, Zilly ruled that the University of Washington had not engaged in "reverse discrimination" against white law school admission aspirants who had not been admitted to the class entering in fall 1994.
[citation needed] In February 2019, Zilly ordered the Department of Defense not to discriminate against U.S. armed forces members who are naturalized citizens by requiring them to undergo periodic security checks.
[8] In September 2020, Zilly denied a government motion to dismiss a civil rights lawsuit filed by the family of Che Andre Taylor, who had been shot dead by Seattle police officers in 2016.