The Postlethwaites were a wealthy Natchez, Miss. family. The founder of the family was Samuel Postlethwaite (ca. 1772-1825), a native of Carlisle, Pa. By 1802 he settled at the Natchez district of Mississippi. In 1803, he married Ann Dunbar, daughter of William "Sir" Dunbar, a wealthy local planter. He handled business affairs of his ailing father-in-law, and upon his death in 1810, acted as an executor of his estate. In 1810, 1811, and 1813, he journeyed up the Red River to supervise the restoration of Dunbar's salt business there. In 1815, Samuel Postlethwaite was one of the incorporators and original stockholders of the Bank of Mississippi; he also was one of the owners of the Natchez Steamboat Company.
William Dunbar Postlethwaite (1810-1863), son of Samuel Postlethwaite and Ann Dunbar Postlethwaite, attended school in Pennsylvania from 1821-1825, and then settled at Westmoreland, La.
From the description of Postlethwaite family papers, ca. 1778-1938 (bulk 1806-1838). (Huntington Library, Art Collections & Botanical Gardens). WorldCat record id: 122354207