McCoy attended the Wilmington (Delaware) Friends School and received a B.F.A. from Cornell University, Ithaca, New York, in 1933. Summers were spent at the American School at Fountainbleau, France, and additional work was done in the studio of Landislas Medgys in Stuttgart, Germany, and with Despojols in Paris. McCoy then studied with N. C. Wyeth in Chadds Ford, Pennsylvania, where he worked alongside Wyeth's youngest son, Andrew. In 1935 McCoy married the elder Wyeth's daughter, Ann.
McCoy's first solo exhibition was held in 1940 at Robert Vose Galleries, Boston. Other exhibitions followed including those at Babcock Galleries, New York (1941); Currier Gallery, Manchester, New Hampshire; Munson Williams Proctor Institute, Utica, New York; the Schenectady (New York) Museum (1959); the Farnsworth Museum, Rockland, Maine; and the Delaware Art Center, Wilmington (1963).
McCoy's mural commissions include one for the DuPont Building at the 1939 New York World's Fair, which now hangs in the lobby of the Nemours Building in Wilmington; and a mural for the Metropolitan Life Insurance building in New York.
McCoy served as director of the Wilmington Society of Fine Arts and taught at the Pennsylvania Academy of the Fine Arts. He lives on a farm in the Brandywine River Valley at Chadds Ford, Pennsylvania, and spends summers at Port Clyde, Maine.
He won the Adolph and Clara Obrig Prize for Weathered in 1951 from the National Academy. He was proposed for membership in the Academy by his brother-in-law, Andrew Wyeth. To qualify as a full Academician he submitted the watercolor Village By the Sea.