Cullen played a feral child in Mad Max Beyond Thunderdome[3] directed by George Miller, Little Pam in A Girl's Own Story by Jane Campion,[4] Frances in the internationally screened television series Hills End,[5] and Alana, the title role in The Girl From Tomorrow[6] and its sequel The Girl from Tomorrow Part II: Tomorrow's End,[7] a popular sci-fi children's television series that spawned books, newsletters, fanclubs and international awards,[8] including nominations for three (AFI) Australian Film Institute Awards.

At 24, Katharine made her professional stage debut as Vera The Opera House for The Sydney Theatre Company in A Month in the Country, directed by Lindy Davies.

Both plays were directed by artistic director of the Ensemble, Sandra Bates, one reviewer stating, "Cullen's portrayal as the outspoken, heartbreakingly brave young girl is magnificent.

Her ability to imbue her character with a sensitive mix of fragile vulnerability overlaid by outspoken confidence is remarkable.

[13] "A special mention must go to Katharine Cullen's Masha, whose unrequited passion for Konstantin brings about some of the funniest and saddest scenes... Cullen's vodka-soaked rant inspired well deserved spontaneous applause.