Williamina Dean, commonly known as Minnie Dean, was born on September 2, 1844, in Greenock, Scotland. Her father, John McCulloch, worked as a railway engineer. Her mother, Elizabeth Swan, passed away from cancer in 1857 when Minnie was just a child. After her mother's death, little is known about Minnie's life until she arrived in New Zealand.

By the early 1860s, Minnie was living in Invercargill, New Zealand, with two young children. She claimed to be the widow of a Tasmanian doctor, but there is no evidence supporting her marital claims. At this time, she was still using her birth name, McCulloch. On June 19, 1872, she married Charles Dean, an innkeeper. The couple lived in Etal Creek, which was a notable stop for travelers heading to the Otago goldfields. However, when the gold rush ended, they faced financial difficulties.

In search of better opportunities, the Dean family moved to Winton, New Zealand. Charles took up pig farming to support the family. In the meantime, Minnie began baby farming, which involved taking in unwanted children for payment. During this period, many women turned to her for help, wanting to place their children in a safe environment. It is estimated that Minnie might have been responsible for as many as nine young children at once. She received payments for their care, either weekly or as a lump sum.

Infant mortality rates were high in New Zealand during this time, with estimates suggesting around 80 to 100 infants died out of every 1,000 births. As a consequence, some of the children in Minnie's care sadly passed away from various illnesses. There were reports of several babies dying while under her supervision. A coroner's inquest looked into these deaths, but Minnie was not held responsible due to the poor hygiene conditions common at that time. However, rumors began to circulate in the community, leading people to mistrust her.

As the years went by, more children under Minnie's care went missing without any explanations. This added to the suspicions surrounding her activities. Laws at the time were lax, allowing her to operate without keeping records of the children she accepted. Due to this lack of regulation, proving any wrongdoing became difficult, even with growing public unease about her practices.

In 1895, Williamina Dean was seen getting on a train with a baby and a hatbox. Later, she was spotted leaving the train without the baby but still carrying the heavy hatbox. This raised suspicions. A woman named Jane Hornsby said she had given her granddaughter, Eva, to Dean. Clothes that belonged to Eva were found at Dean's home; however, Dean could not show authorities the child. A search for Eva along the railway line did not find any trace of her.

Dean was arrested and charged with murder. Police searched her garden and discovered three bodies buried there. Two of these bodies were identified as babies, and the third was a boy believed to be around three years old. An investigation revealed that one baby, Eva, had died from suffocation and another, one-year-old Dorothy Edith Carter, had died from an overdose of laudanum. The cause of death for the third child was not determined.

At her trial, Dean's lawyer argued that all the deaths were accidental and had been hidden to protect Dean from bad publicity. On June 21, 1895, the jury found her guilty of murdering Dorothy Carter and sentenced her to death.

While awaiting execution, Dean wrote an account of her life, claiming she had cared for 28 children in total. She reported that six of these children had died while in her care, but authorities could not account for several other children.

On August 12, 1895, Dean was hanged at the Invercargill prison. She holds the grim distinction of being the only woman ever executed in New Zealand. After her death, new laws related to child protection were established in New Zealand, partly due to the concerns raised by her case. She was buried in Winton alongside her husband who died years later.