Newstead had been put up for sale, but pending completion of the contract was still in his possession. During his last visit but one, whilst his sister was his guest, he became engaged to Miss Anna Isabella Milbanke (b. May 17, 1792; d. May 16, 1860), the only daughter of Sir Ralph Milbanke, Bart., and the Hon. Judith (born Noel), daughter of Lord Wentworth. She was an heiress, and in succession to a peerage in her own right (becoming Baroness Wentworth in 1856). She was a pretty girl of "a perfect figure," highly educated, a mathematician, and, by courtesy, a poetess. She had rejected Byron's first offer, but, believing that her cruelty had broken his heart and that he was an altered man, she was now determined on marriage. High-principled, but self-willed and opinionated, she believed that she held her future in her own hands. On her side there was ambition touched with fancy - on his, a wish to be married and some hope of perhaps finding an escape from himself. The marriage took place at Seaham in Durham on the 2nd of January 1815. Bride and bridegroom spent three months in paying visits, and at the end of March settled at 13 Piccadilly Terrace, London.

Byron was a member of the committee of management of Drury Lane theatre, and devoted much of his time to his professional duties. He wrote but little poetry. Hebrew Melodies (published April 1815), begun at Seaham in October 1814, were finished and given to the musical composer, Isaac Nathan, for publication. The Siege of Corinth and Parasina (published February 7, 1816) were got ready for the press. On the 10th of December Lady Byron gave birth to a daughter christened Augusta Ada. To judge from his letters, for the first weeks or months of his marriage things went smoothly. His wife's impression was that Byron "had avowedly begun his revenge from the first." It is certain that before the child was born his conduct was so harsh, so violent, and so eccentric, that she believed, or tried to persuade herself, that he was mad.

On the 15th of January 1816 Lady Byron left London for her father's house, claimed his protection, and after some hesitation and consultation with her legal advisers demanded a separation from her husband. It is a matter of common knowledge that in 1869 Mrs Beecher Stowe affirmed that Lady Byron expressly told her that Byron was guilty of incest with his half-sister, Mrs Leigh; also that in 1905 the second Lord Lovelace (Lord Byron's grandson) printed a work entitled Astarte which was designed to uphold and prove the truth of this charge. It is a fact that neither Lady Byron nor her advisers supported their demand by this or any other charge of misconduct, but it is also a fact that Lord Byron yielded to the demand reluctantly, under pressure and for large pecuniary considerations. It is a fact that Lady Byron's letters to Mrs Leigh before and after the separation are inconsistent with a knowledge or suspicion of guilt on the part of her sister-in-law, but is also a fact (see Astarte, pp. 142-145) that she signed a document (dated March 14, 1816) to the effect that any renewal of intercourse did not involve and must not be construed as a withdrawal of the charge. It cannot be doubted that Lady Byron's conviction that her husband's relations with his half-sister before his marriage had been of an immoral character was a factor in her demand for a separation, but whether there were other and what issues, and whether Lady Byron's conviction was founded on fact, are questions which have not been finally answered. Lady Byron's charge, as reported by Mrs Beecher Stowe and upheld by the 2nd earl of Lovelace, is "non-proven." Mr Robert Edcome, in Byron: the Last Phase (1909),