Johanna van Warmelo was born on 18 November 1876, to Pastor Nicolaas Jacobus van Warmelo and his second wife Maria Magdalena Elizabeth Maré. Her Father was a Dutch Reformed minister from the Netherlands whilst her mother's family had been early emigrants to southern Africa.

Brandt was educated for two years at the Good Hope Seminary for Young Ladies in Cape Town. When her Father died in 1892, Johanna and her mother set out for a six-month tour of Europe.

At the start of the Second Boer War in 1899, Johanna volunteered along with three of her brothers. She served as a nurse until the British captured Pretoria, the capital of the Transvaal. The Boers did not immediately surrender, however, and a long guerrilla war began.

In 1902 Johanna married a minister, Louis Ernst Brandt. She had become so well known that messages of congratulations came from the Leaders of countries.

Brandt remained involved in South African nationalist politics. When the British declared war on Germany in August 1914, they transferred the garrison in South Africa to the European front. Several Boer officers, led by Lieutenant Colonel Manie Maritz, seized this opportunity to declare South Africa's independence. When the Maritz Rebellion was crushed by the South African government 6 months later, the Nasionale Vroueparty, or National Women's Party, was formed in the Transvaal. Its purpose was to work to free the rebels and to care for their families, as well as to serve as an auxiliary for the National Party. Brandt served as secretary at the party's first congress, held in Johannesburg.

Brandt wrote about revelations that were allegedly made to her on the evening of her mother's death on 7 December 1917 in Pretoria. She published these prophetic revelations in a book called The Millennium in 1918. Her other religious work was the Paraclete, or Coming World Mother which was published in 1936. The works include prophecies for South Africa in which she warns the "tribes" that they must heed their "masters" and of a "dark future". After an alleged angel's revelation in 1916, Brandt is reported to have spoken of South Africans as the chosen race and of Johannesburg being attacked by black people.

In 2000, the South African Post office created a series of stamps about the Writers of the Boer War, including Sir Arthur Conan Doyle and Winston Churchill. Johanna Brandt appears on the 1.30 Rand stamp together with Sol Plaatje and the Anglo-Boer War Medal.