A descendent of William Brewster of the Mayflower, Lydia Sewell painted as a young girl. She began her formal training at the National Academy of Design, where she enrolled in the Antique class in 1876. Additionally Sewell also studied at the Art Students League under William Merritt Chase and William Sartain. and at Cooper Union with Douglas Volk and R. Swain Gifford. Sewell continued her art education in Paris. She attended the Academie Julian, as a pupil of Tony Robert-Fleury and Adolphe William Bouguereau and spent time in the atelier of Charles Emile Carolus-Duran. Sewell first exhibited at the Paris Salon in 1886.
Upon her return, Sewell opened a New York studio, establishing her reputation as a portraitist and decorative painter. Her sitters included Mrs. Flora Bigelow Dodge and Mrs. Peter Cooper Hewett. In 1888, Sewell married the painter Robert Van Vorst Sewell, and the couple made their home at "Fleetwood House," in Oyster Bay, Long Island.
She was one of the founders of the Woman's Art Club, and exhibitied a mural at the Woman's building at the Chicago World's Fair of 1893. Sewell was awarded numerous times during her career, including a bronze medal for her mural at the Fair, and also at the Pan-American Exposition, held in Buffalo in 1901, and the Louisiana Purchase Expoisition, held in 1904. When the Academy eulogized her, they stressed her early development, noting that Sewell's "artistic tendencies were stimulated by the
mountain scenery around her home and before she had received any
instruction she attained considerable facility in the use of color (CM, April 27, 1927).