Thrown together as infants by the marriage of their parents William Godwin and Mary Jane Clairmont in December 1801, it seems inevitable that the two, less than a year apart in age, should have grown up together as codependent, competitive, and sometimes thorns-in-the-side of each other. What Claire lacked in raw literary talent, she made up for by nerve and self-assertion. She accompanied Mary and Percy Bysshe Shelley in their elopement to the continent of 1814, it would appear, on impulse and because, unlike either of the lovers, she was fluent, from her mother's attention to sustaining her heritage, in French. When they returned she spent much of her time in their company, and there is some suspicion that she was herself, at least on an emotional plane, involved with Shelley. In March of 1816, perhaps because she was rebuffed by Shelley, perhaps to show herself the equal of her step-sister, perhaps for the sheer romance of it, she plotted an elaborate subterfuge by which she seduced Lord Byron, then in the final stages of securing a legal separation from his wife: this gave her, she later averred, ten minutes of happiness and thereafter a wholly disrupted existence. Byron made clear to her that she was not to share his life, but when he departed England on 23 April he left behind a distraught eighteen-year-old determined to have it otherwise. Not one to give up without a struggle, somehow Claire managed to convince Shelley and Mary that it was necessary for them all to pursue Byron to his summer residence at Geneva, and nine days later the entourage departed London. By the time they met up with Byron, on 27 May, Claire may have known that she was pregnant; certainly, it was not long after this that it became clear to her. Clear, as well, was the awkwardness of her situation, since Byron, though taking warmly to Shelley and Mary, resolutely refused to have any further relations with Claire and only admitted her into his company when the Shelleys were present. The diplomacy by which Byron was persuaded to accept and provide for the child as his own was left to Shelley.
When the party returned to England at the end of the summer, Claire found lodgings in Bath, where the next January she was delivered of a daughter, whom she named Allegra. The need to get the child under its father's protection partly dictated the next expedition to the Continent, in March of 1818, when the Shelley party went to Italy. Again refusing to see Claire, Byron arranged for Allegra to be brought to him in Venice: there, for a time, the year-and-a-half-year-old infant lived an incongruous existence, charmingly limned in Shelley's "Julian and Maddalo," where she is portrayed as rolling billiard balls around the floor of the Palazzo Mocenigo with Shelley. But Byron was not the type to devote himself to rearing a baby and, to Claire's dismay and the displeasure of the Shelleys, he entered Allegra into a convent school. There she died of typhus in the spring of 1822, with a furious Claire accusing Byron of complicity in her child's murder.
With Shelley's death only two months later, the whole circle fell apart. Mary decided to return to England and from her small store of funds paid for Claire to join her brother Charles in Vienna. From there, in March 1823, with a command of five languages, she moved to Russia to undertake the position of a governess, for the first year in St. Petersburg, then for another four in Moscow. She returned to England in 1828, but after a year was back in Dresden as a governess. In the 1840s she was settled in Paris, from which she made lengthy trips back to England. Finally, in 1870 she moved to Florence, where she died in her eighty-first year. Henry James's novella, The Aspern Papers, is based on the unscrupulous attempts in her last years to wrest from her the Shelley memorabilia she treasured.