How I, then a young girl, came to think of, and to dilate upon, so very hideous an idea?— Mary Shelley
During the rainy summer of 1816, the "Year Without a Summer", the world was locked in a long cold volcanic winter caused by the eruption of Mount Tambora in 1815. Mary Shelley, aged 18, and her lover (and later husband) Percy Bysshe Shelley visited Lord Byron at the Villa Diodati by Lake Geneva in Switzerland. The weather was consistently too cold and dreary that summer to enjoy the outdoor holiday activities they had planned, so the group retired indoors until dawn.
Sitting around a log fire at Byron's villa, the company amused themselves by reading German ghost stories translated into French from the book Fantasmagoriana, then Byron proposed that they "each write a ghost story". Unable to think of a story, young Mary became anxious: "Have you thought of a story? I was asked each morning, and each morning I was forced to reply with a mortifying negative." During one evening in the middle of summer, the discussions turned to the nature of the principle of life. "Perhaps a corpse would be re-animated", Mary noted, "galvanism had given token of such things". It was after midnight before they retired, and unable to sleep, she became possessed by her imagination as she beheld the "grim terrors" of her "waking dream".
I saw the pale student of unhallowed arts kneeling beside the thing he had put together. I saw the hideous phantasm of a man stretched out, and then, on the working of some powerful engine, show signs of life, and stir with an uneasy, half vital motion. Frightful must it be; for supremely frightful would be the effect of any human endeavour to mock the stupendous mechanism of the Creator of the world.
In September 2011, astronomer Donald Olson, after a visit to the Lake Geneva villa the previous year and inspecting data about the motion of the moon and stars, concluded that her "waking dream" took place "between 2 a.m. and 3 a.m." on 16 June 1816, several days after the initial idea by Lord Byron that they each write a ghost story.
She began writing what she assumed would be a short story. With Percy Shelley's encouragement, she expanded the tale into a full-fledged novel. She later described that summer in Switzerland as the moment "when I first stepped out from childhood into life". Shelley wrote the first four chapters in the weeks following the suicide of her half-sister Fanny. This was one of many personal tragedies that impacted Shelley's work. Shelley's first child died in infancy, and when she began composing Frankenstein in 1816, she was likely nursing her second child, who was also dead by the time of Frankenstein's publication.
Byron managed to write just a fragment based on the vampire legends he heard while travelling the Balkans, and from this John Polidori created The Vampyre (1819), the progenitor of the romantic vampire literary genre. Thus two seminal horror tales originated from the conclave.
The group talked about Enlightenment and Counter-Enlightenment ideas as well. Shelley believed the Enlightenment idea that society could progress and grow if political leaders used their powers responsibly; however, she also believed the Romantic ideal that misused power could destroy society.
Shelley wrote much of the book while residing in a lodging house in the centre of Bath in 1816.
Shelley's manuscripts for the first three-volume edition in 1818 (written 1816–1817), as well as the fair copy for her publisher, are now housed in the Bodleian Library in Oxford. The Bodleian acquired the papers in 2004, and they belong now to the Abinger Collection. In 2008, the Bodleian published a new edition of Frankenstein, edited by Charles E. Robinson, that contains comparisons of Mary Shelley's original text with Percy Shelley's additions and interventions alongside.