In 1913 she was appointed to the Melbourne Police Court to assist women and girls in distress.
The stories of her work in the courts are legend. She was never concerned with religious denominations, and she never gave up. She had a wonderfully spontaneous sense of humour and also the knowledge and tenacity of purpose to see a task through, which meant that many of the women and girls who came before the magistrates were able to be assisted and rehabilitated.
The officers of the court and police force placed great reliance on her judgement and many women were kept out of prison by her intercession and acceptance of responsibility for them. From 1917 the matronship of Salvation Army women's shelters was added to her responsibilities.
Mary Anderson was a highly respected Justice of the Peace, was given a Life-Governorship of the Royal Melbourne Hospital, invited to become an Associate Member of the National Council of Women, appointed a Life Member of the Honorary Justices Association, and was the first Vice-President of the Probation Officers Association of Victoria.


Mary Anderson, in 1943, was the first Australian woman to be awarded the Order of the Founder, which is The Salvation Army's highest honour. In more than 100 years it has been awarded fewer than 300 times. The Order was inaugurated in 1920 to recognise Salvationists who had rendered distinguished service, such as would have specially commended itself to the Founder, William Booth. The citation included the following statement:
"... has, for 25 years carried on a sacrificial and remarkable blessed ministry as Police Court Officer in the City of Melbourne, highly appreciated by Magistrates and the public, also powerful in influence among wrongdoers of many degrees."
She retired in 1935 but continued her police work for a further 11 years, and was honoured in the Queen's Birthday Honours of 1956, receiving the MBE from the Governor of Victoria.
Mary Anderson (‘the Little Major’) died in 1956. Newspaper writer E.W. Tipping devoted his whole feature "In Black and White" to a description of her funeral service, and the City of Melbourne paid tribute to a selfless ‘Advocate for the Troubled’.