Grace Cahill was the leader of the Madrigal branch of the Cahill family, a very rich lady, cartographer and frequent aviator in her private plane, the Flying Lemur. Depending on who you asked, she was one of the most loved or hated members of the family. Her death in 2008 was the starting point of a chain of events to find the 39 Clues.
Grace was the daughter of Edith Green, and James Cahill, the younger sister of Beatrice Cahill, the older of Fiske Cahill. She became an avid aviator, flying the Flying Lemur, her own private plane around the world collecting map data in unexplored lands, and mapping them, creating some globes of the world, one she autographed and put in the secret library of her mansion. Grace had a daughter, Hope Cahill with Nathaniel Hartford.
In 1985 Anne Cahill gave Grace an Ekaterina Clue, Phosphorus, to fly to Scotland in resentment of her branch. Grace took the Flying Lemur and briefly landed at a Scottish tarmac, and was contacted by Simon and Jim, air traffic controllers as she flew. Grace was flying over Loch Ness when her plane was attacked by the Ekats, and it began to fall in Loch Ness, but Grace was able to survive. She went to Anne's 'grave', and made a map of the plane crash. She sent a letter to Anne detailing the misfortune.
Nathaniel died on a buisness trip to Moscow, and Hope died in a fire with Arthur Trent, her husband. She had a week-long winter holiday with other members of the Cahill family. She saw her grandchildren, Amy Cahill, and Dan Cahill every weekend after pressuring Beatrice in becoming their legal guardian. Once she found a young dan with an Aztec dagger he stole from her display case and told him about it instead of scolding him. She'd have picnics with Amy, telling her about the Ekaterina, Janus, Tomas, and Lucian branches. Grace didn't permit Attelboro workers to look in her lab, and got sued. Her lawyer William McIntyre convinced the jury to rule in her favor over Attelboro.
She later contracted terminal skin cancer, and the last time she saw Dan and Amy, she told Amy that she'll make her proud. Five minuets before she died with Saladin on her lap she decided to use her alternate will, William McIntyre warning her of the outcome. Grace then passed away. Her body was put outside her mansion.