American poet whose verse, distinctively American in character, was admired in her day.

Sarah Apthorp was the daughter of a well-to-do merchant and evidently acquired an unusually thorough education. In 1781 she married Perez Morton. She had formed the habit of writing verse in childhood, and in 1789 she began contributing to the Seat of the Muses department of the newly established Massachusetts Magazine. Her early poems, ranging in manner from elegy to pastoral, were published under the name Constantia and later Philenia. The work of Philenia soon attracted the notice of domestic and even British critics, who warmly praised her first volume, a long verse narrative entitled Ouabi; or, The Virtues of Nature (1790), a tale of Native Americans that was cast in the “noble savage” mold.

Her verses, which continued to appear in the Columbian Centinel, the New York Magazine, and the Tablet and later in the Port Folio, the Monthly Anthology, and other periodicals, established Philenia as the foremost American woman poet of her period in America. Beacon Hill: A Local Poem, Historic and Descriptive (1797) and its sequel, The Virtues of Society: A Tale Founded on Fact (1799), are consciously American works. Her last published work, My Mind and Its Thoughts, appeared in 1823. In 1837, after her husband's death, she returned to Quincy, where she died in 1846.

For more than a century Morton was falsely believed to have written The Power of Sympathy (1789), the first American novel, because of the similarity of the book's plot to a scandalous tragedy that had occurred in Morton's own life—her husband's affair with her sister, followed by the sister's suicide. In 1894 authorship of the book was fixed upon William Hill Brown (Brown, William Hill), a neighbour of the Mortons.