In the winter of 1810-11, Shelley published a series of anonymous political poems and tracts: Posthumous Fragments of Margaret Nicholson, The Necessity of Atheism (written in collaboration with Hogg) and A Poetical Essay on the Existing State of Things. Shelley mailed The Necessity of Atheism to all the bishops and heads of colleges at Oxford, and he was called to appear before the college's fellows, including the Dean, George Rowley. His refusal, to college authorities, to answer questions regarding whether or not he authored the pamphlet resulted in his expulsion from Oxford on 25 March 1811, along with Hogg. A number of writers have speculated that the expulsion of Shelley and Hogg was politically motivated. Hearing of his son's expulsion, Shelley's father threatened to cut all contact with Shelley unless he agreed to return home and study under tutors appointed by him. Shelley's refusal to do so led to a falling-out with his father.
Four months after being sent down from Oxford, on 28 August 1811, the 19-year-old Shelley eloped to Scotland with the 16-year-old Harriet Westbrook, a pupil at the same boarding school as Shelley's sisters, whom his father had forbidden him to see. Harriet Westbrook had been writing Shelley passionate letters threatening to kill herself because of her unhappiness at the school and at home. Shelley, heartbroken after the failure of his romance with his cousin, Harriet Grove, cut off from his mother and sisters, and convinced he had not long to live, impulsively decided to rescue Westbrook and make her his beneficiary. Westbrook's 28-year-old sister Eliza, to whom Harriet was very close, appears to have encouraged the young girl's infatuation with the future baronet. The Westbrooks pretended to disapprove but secretly encouraged the elopement. Sir Timothy Shelley, however, outraged that his son had married beneath him (Harriet's father, though prosperous, had kept a tavern), revoked Shelley's allowance and refused ever to receive the couple at Field Place. Harriet also insisted that her sister Eliza, whom Shelley detested, live with them. Shelley invited his friend Hogg to share his ménage but asked him to leave when Hogg made advances to Harriet. Shelley was also at this time increasingly involved in an intense platonic relationship with Elizabeth Hitchener, a 28-year-old unmarried schoolteacher of advanced views, with whom he had been corresponding. Hitchener, whom Shelley called the "sister of my soul" and "my second self", became his muse and confidante in the writing of his philosophical poem Queen Mab, a Utopian allegory.