John W. Casilear was apprenticed as an engraver under Peter Maverick in 1826 and studied painting under Asher B. Durand in 1831. In 1840 to 1843 Casilear went abroad to study with Asher B. Durand and John F. Kensett. By 1854 he had gained popularity as a painter and no longer pursued engraving. The Crayon of November 1855 (p. 330) noted that:
Mr. Casilear has a number of fine mountain studies, one of which, a study of Mount Washington, taken from a point nearer than any we have before seen, gives an excellent topographical view of it as well as a very impressive one, grand, lonely and lofty.
He became an associate of the National Academy of Design in 1835 and a member in 1851. He exhibited there from 1833 to 1893. He maintained a studio at West 10th Street, New York City, from 1859 to 1893. He was a member of the Century Association from 1851 to 1893.
He also exhibited at the Appolo Association, the American Art-Union, the Pennsylvania Academy of the Fine Arts, the Brooklyn Art Association, the Boston Art Club, the Washington Art Association, and the Maryland Historical Society.