Katherine Huff O'Neil was born in New Orleans, Louisiana, in 1938. She attended Stanford University, and spent her junior year abroad at the University of Geneva in Switzerland. She graduated with a bachelor's degree in political science in 1960. She worked for the Young Republican Division of the Republican Party in Washington, D.C., until she was accepted to the Harvard University Law School. At Harvard, she met Mike O'Neil, and they married in 1961; they later had two children. After one year at Harvard, O'Neil dropped out and began raising a family. In 1964, the young family moved to Tigard, Oregon, and Mike O'Neil practiced law in Portland. Katherine Huff O'Neil worked as a correspondent for the Community Press and the Oregonian newspaper. She returned to school in 1974, studying law at Northwestern School of Law at Lewis and Clark College and graduating in 1977. In 1975, she and Mike O'Neil divorced. She practiced law in Portland, Oregon, and in 1982, she met John Paul Toby Graff when they both worked at the Schwabe law firm. Together, they formed the law firm Graff & O'Neil in 1986. O'Neil was a founding member of Oregon Women Lawyers. She served as a pro-tem judge on the Multnomah County Circuit Court, argued cases in state and federal courts as a trial and appellate lawyer, and held a seat on the American Bar Association House of Delegates. She was also chair of the Oregon lawyer representatives to the Judicial Conference of the Ninth Circuit Court of Appeals.