Joan Rosanove was educated at Clarendon Ladies’ College and the University of Melbourne, where she completed the articled clerks' course in 1917. She was articled to her father, barrister and solicitor Mark Lazarus, and was admitted to practise in 1919.
In 1923, Rosanove became the first woman to sign the Victorian Bar roll, signifying her intention to work solely as a barrister. But work was scarce, and she was unable to obtain a room in Selborne Chambers, where most of Melbourne's barristers had their offices. She returned to working as a so-called amalgam (a lawyer who practised as both a barrister and a solicitor), as she had before 1923. Now her practice flourished. Like many early women lawyers, she specialised in criminal and family law matters.
Rosanove advocated the appointment of women magistrates and the reform of Australia's restrictive and discriminatory divorce laws. She also served as president of the Soroptimists Club of Melbourne and of the Business and Professional Women’s Club.
In 1949 Rosanove again signed the Bar roll, and this time successfully established herself in practice as a barrister, with a room in Selborne Chambers. Despite her skill and reputation, she was not made a QC until 1965, at the age of 69, leading to suspicion that her earlier applications had been unfairly rejected. She was the first woman to become a QC in Victoria.