Hélène Jégado was born around 1803 in Plouhinec, Morbihan, Brittany, France. After losing her mother at the age of seven, she was sent to live with her aunts, who were servants at the rectory in Bubry. There, she learned domestic skills, particularly cooking, which later facilitated her crimes. She worked in various households across Brittany, including Séglien, Guern, Auray, Hennebont, Locminé, Lorient, Pontivy, and eventually Rennes. Her roles primarily involved domestic service and cooking.​

Jégado's first known series of murders began in 1833 while working for Fr. François Le Drogo in Guern. Between 28 June and 3 October of that year, seven members of the household, including the priest, his elderly parents, and Jégado's visiting sister, died suddenly. The deaths were attributed to natural causes, possibly due to the recent cholera epidemic, and Jégado's apparent grief and pious demeanor deflected suspicion. ​

Over the next several years, Jégado moved between households, leaving a trail of unexplained deaths. Notable incidents include the deaths of Marie-Jeanne Leboucher and her daughter in Locminé, the widow Lorey in the same town, and multiple members of Madame Toussaint's household in 1835. Her modus operandi involved administering arsenic, often referred to as "poudre blanche," into food she prepared. ​

After a hiatus from suspected murders between 1841 and 1849, Jégado resumed her activities in Rennes. In 1850, while employed by law professor Théophile Bidard, two servants, Rose Tessier and Rosalie Sarrazin, fell ill and died under her care. Suspicion arose when Jégado proclaimed her innocence unsolicited, leading to her arrest on 1 July 1851. ​

Subsequent investigations linked her to numerous deaths, but due to the statute of limitations, she was tried only for three murders, three attempted murders, and 11 thefts. Her trial began on 6 December 1851, and she was convicted on 14 December. Jégado was executed by guillotine on 26 February 1852 in Rennes, in front of a large crowd. ​