Penfold joined the Office of Parliamentary Counsel (Australia) in 1977. In 1993 she became the first woman to be appointed First Parliamentary Counsel. Her drafting included legislation for the "Tampa" affair (the Border Protection Bill, rejected by the Senate), workplace relations reforms (the "Workplace Relations Act 1996"), and the constitutional amendments which would have been required to have created an Australian republic in 1999. She was awarded the Public Service Medal in 2000. In 2001 she was the first woman to be appointed a Commonwealth Queen's Counsel.