Harding began performing in the late 2000s through holiday drama courses, then moved on to television roles with an appearance in My Place and a three-episode run in season 3 of Packed to the Rafters. Her first major television role was as Sue Knight in Puberty Blues, one of the two teenage girls who were the main characters of the 1979 novel of the same name. Harding's role in the series led to her receiving the 2013 Logie Award for Most Popular New Female Talent, as well as being nominated for the 2013 Logie Award for Most Outstanding Newcomer, and nominated for the AACTA Award for Best Young Actor at the 2nd AACTA Awards in 2013. In 2017, she appeared in "Arkangel", an episode of the anthology series Black Mirror.

Harding's first television appearance was in 2004, appearing in a clip for Play School's "Through the Window" segment. The clip showed Harding and her two mums on a trip to an amusement park, and attracted controversy from the media and politicians for showing homosexual parents.

Harding is a campaigner for LGBT rights, appearing with her mothers before the 2009 New South Wales parliamentary inquiry into same-sex adoption reform. As of 2012 she was the President of gay rights group "Wear it Purple". She is also credited as one of the authors of the children's book The Rainbow Cubby House, along with her mother Vicki.