De Marco was born of Italian parents; in Paris he served four years as an apprentice in a sculptor's studio and worked for the foundry of Claude Valsuani. When Paul Bartlett died in Paris in 1925, his widow hired de Marco to cast many of her husband's sketches and unfinished works in bronze. De Marco emigrated to the United States where he became a naturalized citizen and, in 1936, married American sculptor Clara Fasano.
He has specialized in the creation of religious sculpture. Among his public works are reliefs of Moses and Saint Louis executed in the late 1940s for the House Chamber of the United States Capitol. He has also produced a number of copper engravings.
De Marco has taught sculpture at Bennett Junior College, Millbrook, New York; at the school of the Museum of Fine Arts, Boston; at Columbia University, New York, and at the Academy school from 1959 to 1968. For works exhibited in its annuals, he has received the Academy's Saltus Gold Medal in 1947, Cannon Prize in 1956, and Watrous Medal in 1966.
