Mary Ann Robson was born on October 31, 1832, in Low Moorsley, County Durham. She was the daughter of Margaret Londsdale and Michael Robson, who worked as a colliery sinker. Mary Ann was baptized at St. Mary's Church in West Rainton on November 11 of the same year. She had a sister named Margaret, who was born in 1834 but died as a baby. She also had a brother, Robert, who was born in 1835.

When Mary Ann was eight years old, her family moved to the village of Murton in County Durham. After the move, in February 1842, her father died in a tragic accident when he fell 150 feet down a mine shaft. His body was returned to the family in a sack labeled 'Property of the South Hetton Coal Company'. This incident left Mary Ann's mother a widow, and they faced eviction from their home because it was tied to her father's job. In 1843, her mother remarried a miner named George Stott.

At the age of 16, Mary Ann left home to work as a nurse in South Hetton for a man named Edward Potter, who managed the Murton colliery. Over the next few years, Mary Ann's younger siblings were sent to boarding school in Darlington. Eventually, she returned to her stepfather's home and trained as a dressmaker.

Mary Ann Cotton married four times and had thirteen children, but many of them died young. It is believed that she poisoned her victims with arsenic to collect life insurance payouts.

After her first husband, William Mowbray, died of what was said to be an intestinal disorder in 1865, Mary Ann collected money from his life insurance. Their children also suffered from various illnesses, and some died shortly after William. Next, she married George Ward in 1865. He died in 1866, and she again claimed insurance money on his life.

Mary Ann worked as a housekeeper for James Robinson, a widower with children. Soon after she arrived, one of the children died, followed by others. Mary Ann’s mother also fell ill and died shortly after Mary visited. Mary Ann married Robinson, but several of their children died in infancy or childhood, including their daughter Margaret Isabella.

In 1870, she married Frederick Cotton. He passed away in December of that year from an illness described as gastric fever. Shortly after his death, Mary Ann moved in with Joseph Nattrass, and they had a romantic relationship. Nattrass soon fell ill and died, adding to the list of people who died while under her care.

In 1872, Mary Ann’s stepson, Charles Edward Cotton, died after she had made comments suggesting she wanted to get rid of him. An investigation was opened when a local official grew suspicious. Tests revealed arsenic in Charles' body, leading to an examination of Mary Ann's past. She was arrested and charged with his murder based on the evidence found.

Mary Ann's trial began in 1873, and despite her defense arguing the cause of death could have been accidental, she was found guilty. She was sentenced to death by hanging, which took place on March 24, 1873.