Justine was born in Sydney, and at the age of seven, whilst attending Woollahra Public School with other up and coming talents like Mouche Phillips and Deni Hines, Clarke began appearing in television commercials, one of which was Arnott's Humphrey B. Bear biscuits. At eleven she played the role of Brigitta in the stage musical, The Sound of Music.
Clarke's first significant acting role was as the character Anna Goanna in the 1985 film Mad Max Beyond Thunderdome. The same year she appeared in the TV series The Maestro's Company and featured in the 1986 mini-series Professor Poopsnaggle's Steam Zeppelin. The following year she made appearances in A Country Practice and Willing and Abel
In 1988, Clarke began an eighteen months role on the soap opera, Home and Away, as one of 17 original cast members, playing the character of Ruth "Roo" Stewart. The character of Roo was reinstated in the cast list, in 2010, but was now portrayed by Georgie Parker, hence along with Ray Meagher's character of Alf Stewart, she is only one of two original characters. She was one of several Home and Away cast-members to star in a stage musical about the soap, which toured the UK in 1991.
Following her departure from Home and Away in 1989, Clarke appeared in the short-lived series Family and Friends before going on to act in several mini-series including Come In Spinner, Golden Fiddles and Tracks of Glory.
In 1991, she formed a Dolly Parton inspired band called the Honky Tonk Angels, which disbanded three years later. In the late '90s she was a backing vocalist in the Sydney band Automatic Cherry, that also featured The Cruel Sea guitarist James Cruickshank. The band released the album Slow Burner in 1997. Clarke is also a jazz vocalist and cabaret singer, popular on the Sydney club circuit.
A popular children's entertainer and presenter on the landmark children's television program Play School, Clarke has releases albums and DVDs of Children's music through ABC Music. Titles include I Like to Sing, Songs to Make You Smile, Great Big World, Pyjama Jam! & the ARIA Award winning A Little Day Out With Justine Clarke.
First time I stepped onto that set I felt like I was a child again and I had climbed into the television! I remember feeling slightly nervous about meeting old pros like Jemima and Big Ted, but they were very warm and welcoming and just the same as they are on the show.
Clarke's film Turning April in 1996 was followed by Blackrock with Heath Ledger in 1997. More recently she has starred in the films Danny Deckchair and Look Both Ways. The role of Meryl Lee in Look Both Ways scored Clarke a nomination for an Australian Film Institute (AFI) Lead Actress award in 2005 and the award for Best Actress at the Mar del Plata International Film Festival.
After three episodes of the series Wildside, she played Dr Samantha O'Hara in 21 episodes of All Saints. She also played the leading role in the Australian medical drama The Surgeon and appeared in the third season of the critically acclaimed Australian TV Drama series Love My Way, as Simone, the estranged sister of Asher Keddie's Julia.
An experienced stage actor, Clarke has worked with the Sydney Theatre Company is productions such as The Man with Five Children, Trelawey of the Wells, Cyrano de Bergerac, The Herbal Bed, "Hedda Gabler" and Stiffs, and for Belvoir Company B in Michael Gow's Toy Symphony.
In 2009, she starred as the main character in the television series for Showcase Tangle.
In 2010, Clarke starred in the short film Peekaboo.
In 2012, she appeared in the television series Woodley.
In 2013 and 2014, she starred in the television series The Time of Our Lives playing Bernadette.
In 2015, she plays as Eve on House Husbands "House Husbands"
Clarke has three children named Josef, Nina and Max with her husband, actor Jack Finsterer.Mad Max Beyond Thunderdome (1985) – Anna Goanna
Danny Deckchair (2003) – Trudy Dunphy
Healing (2014) – Michelle
Maya the Bee (2014) – Miss Cassandra (voice)
A Month of Sundays (2015) – Wendy
Red Dog: True Blue (2016) – Diane Carter