Mailman grew up in Mount Isa in north-west Queensland. She is one of five children born to Wally Mailman, a famous rodeo rider and horseman, and Jane (Heeni) Mailman, the daughter of a preacher and talented musician. She has both Indigenous Australian (Bidjara) and Māori (Ngati Porou and Te Arawa) heritage. In 1992 she graduated from Queensland University of Technology Academy of the Arts with a Bachelor of Arts majoring in Performing Arts. Since then she has worked extensively in Australian film, television and theater as well as many contributions overseas. She played the role of Kate in a La Boite Theatre production of Shakespeare's 'The Taming of the Shrew' in 1994.
In 1998, Mailman won her first AFI win, was for playing the character "Nona", in the Australian independent filmRadiance. From here she has since become one of Australia's most prominent local actors especially through her lead role on The Secret Life of Us, a role for which she was twice awarded Most Outstanding Actress In A Drama Series at the Logies (2002 and 2004). She stayed during the shows entire duration (2001–2006).
She then took part in a four-part television documentary series with Cathy Freeman called Going Bush (2006) where the pair set off on a journey from Broome to Arnhem Land spending time with Indigenous communities along the way. She was featured prominently in the Leah Purcell documentary Black Chicks Talking (2001), where she candidly discussed her thoughts about her Aboriginal heritage.