Born in Cooma, New South Wales, Duncan is known to Australian audiences as a character Actress in television, stage an films, as well as having appeared in television commercials. Australian television roles included several episodes of the Australian series Hunter (1967) and she appeared for several months in the evening soap opera Number 96 playing Helen Sheridan in 1973. From there, a role on the soap Certain Women followed. She later had an ongoing role in Skyways, and guest starred in several episodes of A Country Practice as Rowena Elliott, Terence Elliott's wife. Film roles include playing a lascivious, sadistic and murderous lesbian armed with a crossbow in exploitation film Turkey Shoot (1982).

She later emigrated to the United States where she acted on television. From October 1988 to September 1994, she portrayed Iris Carrington Wheeler on the daytime soap opera Another World. She succeeded Beverlee McKinsey in the role after a long absence. Many of Duncan's critics, including TV Guide's Michael Logan, remarked that her accent sounded less American than her counterparts, making her seem out of place in the serial. As an Actress, Duncan generally used a Cultivated Australian accent, which is similar to Received Pronunciation.

After leaving the Another World role in the mid-1990s, Duncan returned to Australia, where she acted in guest roles on television series and lampooned her former soap opera image on television commercials. In 2000 Duncan was diagnosed and successfully treated for early stage breast cancer. In 2003, she appeared as Anna Denton on CrashBurn. In 2004, Duncan returned to the United States to fill in for Eileen Fulton as Lisa Grimaldi in three episodes of As the World Turns while Fulton was on emergency medical leave. Duncan continues to take permanent residence in Australia.

In 2006, Duncan switched hats from acting to take up a post as fundraising manager for the GO Fund, which is a devoted to research and treatment for gynaecological cancer in New South Wales. Duncan left the GO Fund in 2009. She currently serves as an Ambassador for the Breast Care Centre at the Royal Hospital for Women and as the Ambassador for Tenterfield Shire Council for Australia Day. As well, Duncan is actively involved in the Catholic Women's League of Sydney and campaigns for funding for awareness of Sudden Infant Death Syndrome, having lost her own grandson to the condition.