In 1989 Justice Duffy was appointed by Governor George Deukmejian to the Monterey County Municipal Court, becoming the first woman judge in the history of Monterey County. She handled civil and criminal assignments and founded the Monterey County Drug Treatment Court in collaboration with the Monterey County Departments of Health and Probation. She served as presiding judge of the Municipal Court in 1997 and 1998. When the municipal and superior courts consolidated in 1998, she served as assistant presiding judge of the consolidated courts. Governor Pete Wilson appointed Justice Duffy to the Monterey County Superior Court in 1999 where she served as assistant presiding judge from 1999 to 2002, supervising the court's criminal departments. Justice Duffy was appointed to the Court of Appeal by Governor Arnold Schwarzenegger in May, 2005. The voters of the State of California elected her to a full term in 2006.
Justice Duffy currently serves on the Judicial Council's Advisory Committee on Criminal Jury Instructions. She has been a speaker and panelist for various educational programs for judges, lawyers and law students. She has participated in moot court programs for Santa Clara University School of Law and Monterey College of Law and mock trial competitions in Monterey and Santa Clara Counties. She has been active in various community organizations in Monterey County, including Monterey College of Law, where she served as trustee for six years, and the Salinas Rotary Club.
Justice Duffy and her husband Thomas Duffy have been married since 1977; they have two adult daughters. They are dedicated skiers and long distance cyclists.