Irish writer chiefly remembered for her Conversations of Lord Byron and for her London salon.

Her father sold her into marriage at 15 to Captain Maurice St. Leger Farmer, a sadist from whom she fled after three months. He died in a drunken brawl in 1817, after which Marguerite married Charles Gardiner, Viscount Mountjoy and Earl of Blessington. Of rare beauty, generosity, and wit, Lady Blessington had been painted at 18 by Thomas Lawrence. She formed a brilliant salon and began to write essays and sketches of London life.

In 1822 the Blessingtons went abroad, accompanied by the young Count d'Orsay, who married the earl's daughter by his first wife. They spent two months in Genoa with Byron and lived in Italy and then in France until the earl's death in May 1829. Their extravagant tastes had drained his fortune, and the countess, returning to London accompanied by d'Orsay, whose marriage had broken up and who remained with her all the rest of her life regardless of scandal, began to support herself by writing. Her first novel, Grace Cassidy, or, The Repealers (1833), was a success. Her journals furnished material for Conversations of Lord Byron (1834), The Idler in Italy (1839), and The Idler in France (1841). She wrote several other novels and edited two annuals, The Book of Beauty and The Keepsake, to which she contributed.

In spite of her literary success, Lady Blessington could not stay out of debt, and in April 1849, to avoid ruin, she and d'Orsay fled to Paris, where they remained until their deaths.