Rosie Batty, 52, from Tyabb, a small town in Victoria, began her work after her son Luke, 11, was stabbed to death.
She received the award from Australian Prime Minister Tony Abbott at a ceremony at Parliament House, Canberra.
The award winners are selected from public nominations and announced ahead of Australia Day, on 26 January.
The 2015 Young Australian of the Year is 21-year-old West Australian Drisana Levitzke-Gray, a young deaf woman who advocates for other deaf people.
Senior Australian of the Year is 61-year-old Jackie French from New South Wales, a children's author and advocate for children with learning difficulties, and the Australian Local Hero is 41-year-old Juliette Wright from Queensland, who established an online platform to get goods and services to vulnerable and marginalised people.
This year was the first time in the 55-year history of the awards that women have secured the top honour in all four categories.
The recipients were great Australians and great role models, said chairman of the National Australia Day Council, Ben Roberts-Smith.
"It is a great moment in the 55-year history of these awards to honour four women.
"Rosie, Jackie, Drisana and Juliette remind us of the many ways in which women contribute to our nation; that women are a force for change, a voice for rights, influencers, educators and the heart of our communities," he said.
After her son's death, UK-born Ms Batty emerged as an articulate and powerful advocate for the rights of women and children living in violent relationships, giving new force to efforts to prevent family violence across Australia.
Family violence activists say Ms Batty's ability to explain to the public why so many women struggle to protect their children from violent partners helped make the issue of family violence a key campaign issue for all political parties in last November's Victoria state election.
Greg Anderson, 54, killed Luke Batty in front of shocked onlookers at a cricket oval in Tyabb.
The father and son had been playing together during an access visit in February 2014 when Anderson suddenly struck his son with a cricket bat and stabbed him.
Anderson was shot by police after threatening them with a knife when they tried to detain him. He died later in hospital.
Ms Batty was at the cricket ground when the attack happened.
The public killing shocked Australians in a way that many other acts of family violence committed every week in the country have not.

The pedestrian was pronounced dead at the scene of the crash, which happened at 01:35 BST on High Road, at the junction with Seven Sisters, in Tottenham.
His next of kin have been told, the Met said.
A 36-year-old man has been arrested on suspicion of failing to stop at the scene and causing death by dangerous driving.
He is in custody for questioning.
Road closures are in place and parts of the pavement between the High Road and Stonebridge Estate are also closed.

Robert White tried to make his pet cat Salem inhale drugs on 16 May, before he stubbed it out on the animal.
Birmingham Magistrates' Court heard the kitten suffered third-degree burns to 80% of its face, and after being taken to the vets he was put to sleep to prevent further suffering.
White, 38, of Rogerfield Road in Birmingham, was handed a four-month sentence suspended for 18 months.
He was also banned from owning pets for life, ordered to pay Â£350 costs and a Â£150 victim surcharge and will be made to attend a drug and alcohol rehabilitation course.
More on this and other stories from Birmingham and Black Country
RSPCA inspector Beth Boyd said the case was "one of the worst" she had investigated.
"I can't even think about what Salem must have gone through that night - he would have been in severe pain, as well as being very frightened," she said.
"At least now some justice has been done for him."
A woman who became a campaigner against domestic violence after her son was killed by his father last February has been named Australian of the Year.
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