The fourth canto of Childe Harold was published on the 18th of April 1818. Nearly three months went by before Murray wrote to him, and he began to think that his new poem was a failure. Meanwhile he completed an "Ode on Venice," in which he laments her apathy and decay, and contrasts the tyranny of the Old World with the new birth of freedom in America. In September he began Don Juan. His own account of the inception of his last and greatest work is characteristic but misleading. He says (September 9) that his new poem is to be in the style of Beppo, and is "meant to be a little quietly facetious about everything." A year later (August 12, 1819), he says that he neither has nor had a plan--but that "he had or has materials." By materials he means books, such as Dalzell's Shipwrecks and Disasters at Sea, or de Castelnau's Histoire de la nouvelle Russie, &c., which might be regarded as poetry in the rough. The dedication to Robert Southey (not published till 1833) is a prologue to the play. The "Lakers" had given samples of their poetry, their politics and their morals, and now it was his turn to speak and to speak out. He too would write "An Excursion." He doubted that Don Juan might be "too free for these modest days." It was too free for the public, for his publisher, even for his mistress; and the "building up of the drama," as Shelley puts it, was a slow and gradual process. Cantos I., II. were published (4to) on the 15th of July 1819; Cantos III., IV., V., finished in November 1820, were not published till the 8th of August 1821. Cantos VI.-XVI., written between June 1822 and March 1823, were published at intervals between the 15th of July 1823 and the 26th of March 1824. Canto XVII. was begun in May 1823, but was never finished. A fragment of fourteen stanzas, found in his room at Missolonghi, was first published in 1903.

He did not put all his materials into Don Juan. "Mazeppa, a tale of the Russian Ukraine," based on a passage in Voltaire's Charles XII., was finished by the 30th of September 1818 and published with "An Ode" (on Venice) on the 28th of June 1819. In the spring of 1819 Byron met in Venice, and formed a connexion with an Italian lady of rank, Teresa (born Gamba), wife of the Cavaliere Guiccioli. She was young and beautiful, well-read and accomplished. Married at sixteen to a man nearly four times her age, she fell in love with Byron at first sight, soon became and for nearly four years remained his mistress. A good and true wife to him in all but name, she won from Byron ample devotion and prolonged constancy. Her volume of Recollections (Lord Byron juge par les temoins de sa vie, 1869), taken for what it is worth, is testimony in Byron's favour.