Italian was her first language and the language in which she corresponded with her father. She studied ballet while she was a teenager, becoming good enough to train with Isadora Duncan. She started to show signs of mental illness in 1930, around the time she began casually dating Samuel Beckett. Her deteriorating mental state caused him to call off the relationship, and in 1934, Carl Gustav Jung took her in as a patient. Soon after, she was diagnosed with schizophrenia at the Burghölzli psychiatric clinic in Zurich. She was placed in an institution in Ivry-sur-Seine, France, in 1935.
Her mental state, and documentation pertaining thereto, is the subject of a recent study by Carol Shloss, who believes Lucia to have been her father's muse for Finnegans Wake. The study makes heavy reference to the letters between Lucia Joyce and her father, and became the subject of a copyright misuse suit by the James Joyce estate. On 25 March 2007, this litigation was resolved. Her life was the subject in 2004 of a West End play, Calico, written by Michael Hastings, and in the 2012 graphic novel, Dotter of Her Father's Eyes.