Percy Bysshe Shelley was born 4 August 1792 at Field Place, near Horsham in Sussex, the eldest son of Sir Timothy and Elizabeth Shelley. His first two volumes of poetry were published at his fathers expense and co-authored with his sister Elizabeth.

In August 1811, he eloped with Harriet Westbrook, who as the daughter of a coffee-house owner was of a decidedly inferior station to the aristocratic lineage the Shelley family claimed. Although Sir Timothy settled enough income on the young couple for their modest comfort, he refused ever to see his son again, instead handling all their relations through his London solicitor. By 1813, Shelley had settled in London, where he printed his first major poem, Queen Mab. In June of that year, Harriet gave birth to their daughter Ianthe. As the year wore on, however, relations between Harriet and Shelley deteriorated seriously, and Shelley came to regret the impulsiveness of his marriage. It was in this state of frustration and emotional duress early in 1814 that Shelley, on a visit to Godwin, became reacquainted with his sixteen-year old daughter Mary, herself just returned from a prolonged stay in Scotland. Soon the two of them had fallen in love. On 27 July 1814, they fled to the continent along with Mary's step-sister Claire Clairmont, and for the ensuing month-and-a half they traveled through France, Switzerland, and Germany in a journey later memorialized as a History of a Six Weeks' Tour.Upon their return in September harsh reality quickly intruded on the couple's idyll. Shelley found himself shunned by Godwin and dunned by creditors. With Mary now pregnant, Harriet Shelley gave birth to a second child, whom she named Charles, on 30 November 1814.In January of 1816 Mary gave birth to a son, whom she named William after her father. Later that spring Shelley, Mary, and Claire once more set out for the continent, this time rather from necessity.During March Claire, through an improbable but successful act of rivalry with Mary, had managed to seduce Lord Byron. Her liaison resulted in a pregnancy. This excursion to Geneva, then, was assumed so as to acquaint Byron first-hand with the necessity of providing for his future child.

Although Byron refused to meet with Claire, he readily gave assurance for this provision, and during the several months the party remained at Geneva, he and Shelley became close friends. During the summer of 1816, while Byron finished Childe Harold's Pilgrimage, Canto 3, Mary set to work on Frankenstein. Shelley wrote surprisingly little, but in Byron's company seems to have discovered his own distinctive stylistic voice, which can be readily discerned in the two poems he then composed, the "Hymn to Intellectual Beauty" and "Mont Blanc." On 29 August, the day before Mary's nineteenth birthday, the Shelley party left for home, carrying the manuscript of Byron's poem for delivery in London.

Shelley drowned in the Mediterranean Sea on 8 July 1822. After his body washed ashore near Viareggio, it was cremated according to the dictates of Italian law. His ashes were buried in the Protestant Cemetery (actually, Cimitero Acattolico or non-Catholic Cemetery) in Rome. In 1854, three years after Mary's death a monument was erected in memory of both the Shelleys.