Prior to entering the Legislative Assembly of Victoria, Hennessy practised as a solicitor specialising in personal injury and employment law, and was a senior advisor to former Victorian premier Steve Bracks.
[2] A former candidate for preselection for the federal seats of Holt and Isaacs, Hennessy is a former State President of the Victorian Labor Party.
[3] When state minister and Altona MLA Lynne Kosky resigned early in 2010, Hennessy won endorsement for Labor preselection.
During her tenure as Attorney-General, Hennessy worked on reforms relating to voluntary assisted dying, safe access for women's health services, childhood vaccination, medicinal cannabis and wage theft, as well as reforms to decriminalise public drunkenness, a spent convictions scheme and legislation banning gay conversion being introduced to parliament.
[5] Hennessy was also Attorney-General during the Royal Commission into the Management of Police Informants and, before the report of the Royal Commission was released, Hennessy referred the case of Faruk Orman to the Court of Appeal citing "credible evidence that there may have been a miscarriage of justice in Mr Orman's case arising from Nicola Gobbo's conduct and use as a human source by Victoria Police.